So there are three typings that I read here that interested me more than I thought they would, and then I have one more submission of my own to make after. I won't be explicitly quoting or answering Quziel's questions, but they are all kept in mind when working on these, particularly questions C through E.
---------------------------------------------------
One typing that I really liked the sound of here was dex18 's Ground/Flying. I initially thought that Ground would be a rather poor typing to use alongside Color Change due to its poor weakness/resistance distribution w.r.t. self-resisting types, but thinking about this one feels as though its a nice hybrid that makes the best of a few scenarios. The biggest red light is that big 4x weakness to Ice, but looking at who we can or cannot safely switch in against at the moment, this only really effects two choice locked Pokemon, Kyurem and Syclant. At the cost of turning these matchups into mostly loses, we gain so much more.
One of the big threats of coming in against fighting types was the follow up from flying attacks from foes like Tomohawk, Tornadus, or even Hawlucha. The resist from Fighting paired with an enemy flying move sending us back to Flying will allow us time to set up and then take them out with our own SE Flying STAB. Likewise, the usually-neutral ground type that was extremely common is no longer a threat to us at all thanks to our immunity. The enemy is either forced to switch or to go through hoops just to hit us with a ground move, at which point we will have had ample time to set up and start sweeping.
The aforementioned Ground immunity gives us a significant amount of safe switchins against our currently neutral pool of mons like Landorus, Hippodown, Colossoil, or even Sheer Force Nidoking. Likewise, the combo of Volt Switch -> SE Ground is a non-problem since we are immune to the Volt Switch entirely. The only noteably dangerous switch ins we have are the universal CC bullies like Cinderace/Dragapult/Venusaur, Choice Ice Kyurem/Syclant, and water types that can follow up with SE Grass/Electric moves like Krilowatt (which are already a threat). In exchange we have a mountain of switch in routines that either let us setup or change our type to a far-less-punishable monotype thanks to Color Change. It is extremely unlikely that we can be revenge killed by Ice type thanks to Color Change, nullifying this typing's greatest weakness entirely thanks to our ability.
---------------------------------------------------
The other typing here I think suits us really well is G-Luke 's Fairy/Flying typing. This typing has 4 self-resistant weaknesses and Rocks, which does put some strain on stats stage, or on partners that it wants to play alongside. For instance, a screen setter like Astrolotl is typically weak to Dragon or Ground moves, but being able to pivot into our Fairy/Flying Color Changer means that we would absorb any Dragon or Ground moves as immunities. Likewise, the screens severely downplay the threat of the 2x effective switch in moves, whose self-resistance means that they become 1/4 effective compared to their first hit.
Even without a partner, we can switch in extremely safely against the common Fighting or Ground moves that normally threaten color change thanks to our immunity and 0.25x Fighting move multiplier. Damage taken from that first Fighting move is extremely negligible that it would be as if we switched in for free, to the point where even the followup SE Flying/Fairy/Psychic move isn't a death sentence, and we can still safely setup.
The biggest flaw is that this is rocks weak, it is otherwise a perfect typing for a Color Change Setup Sweeper. In exchange, we have an immunity to all other hazards such as Spikes.
---------------------------------------------------
The last one I was waiting to see submitted was Ghost/Electric, submitted by viol and bass. This combo has a few interesting combinations I like. Like the other two types I have here, it can switch in and setup safely against Fighting moves that normally punish Color Change thanks to its total immunity. Most of what I have to say was summarized by them in regards to how it forces extremely popular threats that typically bully Color Change like Tomohawk into submission without giving up on any of our good matchups other than arguably Hydreigon.
Being Knock weak sucks, and Close Combat followups can still hurt after a Knock Off. But this would more often than not come up after we have time to setup, and against mons that only have STAB from either 1 (Colossoil) or neither (Zeraora) of the moves. This typing leaves us with a lot of versatility in the movepool stage while also giving us a number of switch ins against Fighting/Electric moves where we keep our threatening STAB combo. This makes us very useful at not just checking threats that dislike Color Change, but also scaring a significant amount of normally frightening threats from Moltres to Dragapult. This pairs perfectly with any number of Pokemon that are afraid of Fighting moves (Equilibra, Blissey, even our own Hydreigon) generating a lot of safe switchins. Flying types also make for fantastic partners, as the opponent needs to make a decision between using an uneffective electric move against our Color Changer, or an uneffective Ground move against the original flier/levitater. (Again, Equilibra.)
---------------------------------------------------
I also wanted to submit Electric/Flying as I feel it has a lot of relevant merit as well. I'll keep it a bit shorter compared to the Fairy/Poison post I made since this is already a very wordy post. Electric/Flying is similar to Ground/Flying as discussed above where it has a glaring weakness in Ice (albeit 2x instead of 4x.) Its only other weakness is to Stealth Rocks, like Fairy/Flying it trades for immunity to all other hazards. Both are also uncommon typings to see in general, meaning that we will be mostly taking neutral damage before changing type. Mostly.
There is one major interaction we have, which is with the Flying type. Because we retain our typing when getting by Flying moves, we turn that typing into a self-resistant typing. In other words, Hurricane spam that normally punishes this ability can't do that anymore. Additionally, we retain our Electric STAB, allowing us to immediately punish with an SE Electric move of our own. The only flying type this combo fails to work against is Cawmodore, but that's not exactly something we would switch into and survive to begin with.
Beyond that, everything I said about ground moves with the last two Flying types applies here as well, turning the incredibly common threat into a non-problem. The Volt Switch into Ground spam is also impossible since we keep our Ground immunity. Also I guess we are immune to paralysis, which is nice.
---------------------------------------------------
One typing that I really liked the sound of here was dex18 's Ground/Flying. I initially thought that Ground would be a rather poor typing to use alongside Color Change due to its poor weakness/resistance distribution w.r.t. self-resisting types, but thinking about this one feels as though its a nice hybrid that makes the best of a few scenarios. The biggest red light is that big 4x weakness to Ice, but looking at who we can or cannot safely switch in against at the moment, this only really effects two choice locked Pokemon, Kyurem and Syclant. At the cost of turning these matchups into mostly loses, we gain so much more.
One of the big threats of coming in against fighting types was the follow up from flying attacks from foes like Tomohawk, Tornadus, or even Hawlucha. The resist from Fighting paired with an enemy flying move sending us back to Flying will allow us time to set up and then take them out with our own SE Flying STAB. Likewise, the usually-neutral ground type that was extremely common is no longer a threat to us at all thanks to our immunity. The enemy is either forced to switch or to go through hoops just to hit us with a ground move, at which point we will have had ample time to set up and start sweeping.
The aforementioned Ground immunity gives us a significant amount of safe switchins against our currently neutral pool of mons like Landorus, Hippodown, Colossoil, or even Sheer Force Nidoking. Likewise, the combo of Volt Switch -> SE Ground is a non-problem since we are immune to the Volt Switch entirely. The only noteably dangerous switch ins we have are the universal CC bullies like Cinderace/Dragapult/Venusaur, Choice Ice Kyurem/Syclant, and water types that can follow up with SE Grass/Electric moves like Krilowatt (which are already a threat). In exchange we have a mountain of switch in routines that either let us setup or change our type to a far-less-punishable monotype thanks to Color Change. It is extremely unlikely that we can be revenge killed by Ice type thanks to Color Change, nullifying this typing's greatest weakness entirely thanks to our ability.
---------------------------------------------------
The other typing here I think suits us really well is G-Luke 's Fairy/Flying typing. This typing has 4 self-resistant weaknesses and Rocks, which does put some strain on stats stage, or on partners that it wants to play alongside. For instance, a screen setter like Astrolotl is typically weak to Dragon or Ground moves, but being able to pivot into our Fairy/Flying Color Changer means that we would absorb any Dragon or Ground moves as immunities. Likewise, the screens severely downplay the threat of the 2x effective switch in moves, whose self-resistance means that they become 1/4 effective compared to their first hit.
Even without a partner, we can switch in extremely safely against the common Fighting or Ground moves that normally threaten color change thanks to our immunity and 0.25x Fighting move multiplier. Damage taken from that first Fighting move is extremely negligible that it would be as if we switched in for free, to the point where even the followup SE Flying/Fairy/Psychic move isn't a death sentence, and we can still safely setup.
The biggest flaw is that this is rocks weak, it is otherwise a perfect typing for a Color Change Setup Sweeper. In exchange, we have an immunity to all other hazards such as Spikes.
---------------------------------------------------
The last one I was waiting to see submitted was Ghost/Electric, submitted by viol and bass. This combo has a few interesting combinations I like. Like the other two types I have here, it can switch in and setup safely against Fighting moves that normally punish Color Change thanks to its total immunity. Most of what I have to say was summarized by them in regards to how it forces extremely popular threats that typically bully Color Change like Tomohawk into submission without giving up on any of our good matchups other than arguably Hydreigon.
Being Knock weak sucks, and Close Combat followups can still hurt after a Knock Off. But this would more often than not come up after we have time to setup, and against mons that only have STAB from either 1 (Colossoil) or neither (Zeraora) of the moves. This typing leaves us with a lot of versatility in the movepool stage while also giving us a number of switch ins against Fighting/Electric moves where we keep our threatening STAB combo. This makes us very useful at not just checking threats that dislike Color Change, but also scaring a significant amount of normally frightening threats from Moltres to Dragapult. This pairs perfectly with any number of Pokemon that are afraid of Fighting moves (Equilibra, Blissey, even our own Hydreigon) generating a lot of safe switchins. Flying types also make for fantastic partners, as the opponent needs to make a decision between using an uneffective electric move against our Color Changer, or an uneffective Ground move against the original flier/levitater. (Again, Equilibra.)
---------------------------------------------------
I also wanted to submit Electric/Flying as I feel it has a lot of relevant merit as well. I'll keep it a bit shorter compared to the Fairy/Poison post I made since this is already a very wordy post. Electric/Flying is similar to Ground/Flying as discussed above where it has a glaring weakness in Ice (albeit 2x instead of 4x.) Its only other weakness is to Stealth Rocks, like Fairy/Flying it trades for immunity to all other hazards. Both are also uncommon typings to see in general, meaning that we will be mostly taking neutral damage before changing type. Mostly.
There is one major interaction we have, which is with the Flying type. Because we retain our typing when getting by Flying moves, we turn that typing into a self-resistant typing. In other words, Hurricane spam that normally punishes this ability can't do that anymore. Additionally, we retain our Electric STAB, allowing us to immediately punish with an SE Electric move of our own. The only flying type this combo fails to work against is Cawmodore, but that's not exactly something we would switch into and survive to begin with.
Beyond that, everything I said about ground moves with the last two Flying types applies here as well, turning the incredibly common threat into a non-problem. The Volt Switch into Ground spam is also impossible since we keep our Ground immunity. Also I guess we are immune to paralysis, which is nice.