Regarding the zero knowledge proof, there'd be a chance that the two random ones you reveal are the same color, if three weren't enough.

So by doing this over and over again, if the two you choose are always different colors, you approach a 100% certainty that it's legit.

You never really get a 100% proof but the more times you repeat the closer you are to being sure. At 99.999999% after repeating this enough times, you'd most likely be satisfied.