There is an old fashioned idea floating around that everyone in the picture needs to match. This is not true! Choose colors that look nice together, but don’t fall into the trap that you all need to be wearing the exact same outfit or color. Coordinating is far better than matching. Your photos will be far more interesting with some variation!
A great resource for finding the perfect coordination and colors, besides your photographer, is Pinterest! Depending on your session type, you can find different variations of outfits and colors and even find some links that lead you straight to a website to by those specific items. Some keywords tips would base around the season of your session, and if there will be props in your set.