I would start by considering small cases. First of all there's no use guessing a colour that was not called, since there are infinite of those. So suppose there's one ball. Then you guess the colour that was called and have a 30% chance of being right. Now suppose there are two balls selected. Suppose two different colours were called. You guess one of them. Then you have a 15% chance of being right, because the ball whose colour was called has to be selected (50%) and the caller had to be telling the truth (30%). All other probabilities are effectively 0 because of the infinite possibilities. Now suppose the caller called the same colour twice in a row. Now you have a 30% chance. And so on. So just guess the colour that was called most often (or if more than one, just choose one arbitrarily), then suppose it was called k out of n times, then you have a (k/n) * 0.3 probability of being right.