I don't think she has the third eye because of not fusing completely together but because it's the future vision eye and is unlike a regular seeing eye. But the rest of it makes a lot of sense! << would make sense
Artists and design experts know that triad colours work together in harmony. Triad colour scheme works by showing people the opposite colours on the wheel work together to make all the colours stand out.
That explains why Peridot didn't summon her weapon when her and Steven were cornered by the mini clusters, she didn't know how because she was never taught! That bothered me so much while watching that episode.