If it's for someone in particular, I go with their likes or colors they're using in whatever room the item is for. I recently finished a poppy lap quilt for my mom and the red matches her couch exactly! I was so tickled to see that. Right now I'm working on a comfort quilt for my niece, she likes frogs and bright pinks. I found a cute frog fabric at Joann's so that's my focus fabric and am using several other colors as well as a bright pink.
If I'm doing something "just because", I usually try to find one fabric (in my stash or at the store) that I really like and that's my starting point. Sometimes - unfortunately for my budget - I find a fabric I fall in love with, buy some yardage and then find I have very little that goes with it in my stash...so it's back to the store for me! LOL