The first three colors are the easiest to choose on any project. For the rest, I learned, while working part time in a fabric retail outlet, these guidelines to help speed up the choices:
Choose prints, plaids, medley colors that related to those main three--don't expect to have all three in each, just one or two--that can be the same, darker or lighter in color intensity.
If you need three more than the basic choices, lay out four or five, then eliminate that one or two that obviously don't add spice to the originals. The more colors you need beyond the original choices the more "leeway" is expected, but contrast (dark vs. light, bright vs. light) become more important.
You might be able to make all selections in one shopping trip, but do not expect to do so. Staple a tiny piece of each choice to a card to carry with you to the second or even a third shop. Purchase one more piece/color than you really need for the project. You may be surprised which of all your choices you may end up using.
Don't spend a lot of time agonizing over selections. Your natural first rections will be good. You will have a "feel" when a blue (or a cool tone) is needed within a selection, or when two prints are "too busy"
Ask another customer to choose which of two she prefers...but she will confirm your thinking