the colors can be unexpected, they aren't linear
for example ruby (a red gem) and sapphire (a blue gem) can fuse into a purplish red gem, then fuse with a pink gem to make an orange gem
and like i've said before, i don't buy the "choose their name" thing, i'm confident if they were to show crystal gem ruby and pearl fusing, the fusion will be called rhodonite, however since we will almost definitely never see this, it won't be important in the series