Sometimes when I want to make a colorwork pattern I have a hard time deciding what colors to put together. A two color fair isle sweater looks great in black and white, or most any color and white. What if I don't want white? What if the yarn I like doesn't come in the colors suggested in the pattern? What if the colors I choose are great, but the garment doesn't go with anything I already own?
Different colorwork patterns require different color relationships. A two color geometric might need contrast, but an intarsia flower may do well in more similar color values. Maybe you need funky colors, maybe traditional. One thing that helps me is to use colors from something I own. A bookcover may have a terrific two-color contrast, a teacup may have a nice group of colors; even furniture upholstery works beautifully. Or, just break out the crayons. Really! They're portable, so find the colors you like and take them with you to the shop.
The yarn store staff is usually happy to help. Ask if you can return or exchange any unused skeins if you change your mind. Color is one of the great joys of knitting. Go get some!