The first ~200 odd chapters he wore blue shirts under his suit jacket. And Usopp had his yellow bandana, Luffy his red vest, Chopper’s pink hat, Robin her purpleish cowboy getup, Zoro‘s green boots and haramaki, Nami’s miniskirt was orange at first.
After a while Oda will change things up and start giving them different clothes and colouring, but generally speaking the colour will be based on hair and/or something they wore when they first appeared. Probably because that’ll be, marketing wise, the outfit that is used most throughout all the One Piece multimedia.
White’s the colour I’d guess for Yamato myself. Colour of her top thing, colour of most of her hair. Don’t think she’ll get a shade of blue (there’s already two of them, and he refused to make the very blue Jinbe a third) and red is Luffy‘s colour, which won’t be shared even if it’s a separate shade.