This is CAP. If thats a problem, lets make a Water/Steel with Levitate, for example.
I'd rather see colour change as an ability rather than Multitype, as with a fair amount of bulk, it could counter most types apart from dragon, but it could be fast enough/strong enough to KO it with Ice Beam/Shard before it takes another hit.
That's the problem with Color Change though--it does nothing at all to Ghost and Dragon-types without a very large amount of bulk and ability to hit them back hard, instead only making the problem worse. It also completely ignores the fact that Pokemon use movesets with moves of multiple-types. For example, Starmie can use Surf on it for a potentially neutral hit or whatever to start off, and then go for super-effective with Thunderbolt. Metagross can attack with Meteor Mash, and then Earthquake. Ttar can use Stone Edge, followed by Earthquake. Gyarados can use Earthquake, followed by Waterfall. Zapdos can use HP Grass, and then Heat Wave. Etc. The ability is just extremely easy to exploit by a huge number of Pokemon.
Due to that, Color Change with another problem... having to survive that first hit, preferably with enough health left over to take another one or two hits, or otherwise being able to destroy the opposing Pokemon after the first hit, so that it can't simply be set-up on.
Not to mention it's completely unreliable and unpredictable how things will work out for a Color Change user during battle. After all, you'll be constantly changing type, making STAB moves pretty worthless as it will practically never be its original type. Thus, you have to work under the assumption that it will never have STAB to do what it needs to do when its out, which would increase the amount of strength it needs to do the job.
All in all, Color Change just seems like a horrible ability to me.
Personally though, I really like Multitype, as it pretty much fulfills the concept in a nutshell--it allows CAP10 to handle certain batches of Pokemon with a certain Plate/moveset, but not more than that, as it can only be holding one plate/have one moveset at a time. Thus, CAP10 will be extremely flexible in the number of Pokemon it can counter overall, but yet would have to be made specific to only handle a small number of those Pokemon... just exactly as the concept entails.
Now then, as for some of the concerns I've been seeing for Multitype. First off, yes, it would need a huge movepool. But then again, so do most of these ideas, as in order to be able to capably handle such a wide array of Pokemon as CAP10 is meant to, it needs one. However, that's not necessarily a problem, as a movepool in of itself does not mean anything--just look at Smeargle. Things like the stats are important too. Plus, as is being proposed for a lot of these ideas, we can keep the movepool from being too huge by making good use of Egg Moves and the like, forcing CAP10 to choose just the moves it needs and not have access to some of the rest of its more powerful moves.
Then, there's this conception of Multitype being an "Uber Ability" or something to that effect. However, I disagree with this. Yes, the only current user of this ability is Uber... However, that doesn't make the ability itself broken. I mean, Arceus also has a 720 BST, coming from having 120 in all stats, something that this CAP will never get. We've been quite capable in making our stat-spreads unbroken, and I don't think we'd fail here.
Plus, let's look at what exactly it is Multitype does. It just allows a Pokemon to pick which type it wants to be prior to entering battle. However, that's it--it doesn't let it change it's type mid-battle at the user's whim or anything--all it does is let you pick the Pokemon's type. And since no types are inherently broken, this really shouldn't be an issue at all. Plus, to pick a type, you have to attach a Plate--that means no Leftovers, Life Orb, Choice Specs, Choice Band, etc, on non-Normal sets, limiting it's offensive and defensive abilities on such sets. Thus, what it can do is really dependent upon its stats, ultimately.
Which leads me back to my prior point--as long as we don't give it Arceus-esque BST and stat distribution (which we won't), there shouldn't be a problem, as it needs that to make up for the fact that it can't hold other items enough to push it into being Uber. Without such crazy stats, I severely doubt that Multitype, even when coupled with a large movepool, will push CAP10 into being broken.
Multitype definitely has strengths, but it also has flaws, which helps to keep it in check. It's also a great way to fulfill the concept, one of the best I can think of, as it pretty much is the concept in a nutshell, in my opinion. Thus, I would be more than fine with us choosing to go with it.