With strings I've done some that are totally scrappy and some that are limited (i.e. blues & yellows, pink,white & black or all brights). I love string quilts and think you can mix pretty much anything. Sometimes I will make the center strip one color or fabric to help kind of "tame" them. With other scrappy quilts, like MTS said, you have to watch your value or the pattern can get muddled together. I sometimes avoid medium values and just go dark & light if I don't want to pay too much attention while piecing.