I would recommend getting The Knot planning binder! They have a whole page of color swatch pop-outs that have recommendations on the back of what other colors they go with and then another page where you can put them side by side for different color schemes and decide on your favorite! On Amazon it's a little pricey, but my sister found it in a book store for $20
Maybe start with a color she always wanted her bridemaids in or fiance wanted groomsmen and go from there. My fiance wanted the guys to wear blue so I went off of that as well as the vision/"theme" I want.