Choose a group of possibles, stack them up at eye level - the top of other fabric works - stand back and squint. You'll get a feel for what they will look like together. I miss being able to do that when buying online.
Don't try to be too matchy, matchy. I just freaked out the lqs ladies when I bought a green for the sashing for a special quilt I am planning. The stack said it needed a new color and green goes with so many colors. The quilt will be fuchia, peachy, rose, with a black patterned floral for the border. The rose is the one I'm concerned about, but I am taking my own advice and not being too matching. My oldest daughter does beautiful work, but has trouble choosing fabrics because 'they don't match'. For most people, it comes with time.