For me, the process usually starts with a pattern, and the pattern guides the choice of fabrics.
I found Jinny Beyer's method of choosing fabric helpful (which she talks about in her book on hand quilting). She says, make sure you have several shades of each color you want to work with, have a color that is darker than all the others, have a color that is brighter than all the others, and include some neutrals. Going by that, I would say, maybe add a black to your colors as the neutral, and some lighter shades of blue and purple.