Many years ago in an art class, the instructor offered this tip if one is confounded by combining colors. "Go look at a red rose and then determine if red and greeen don't go together."
What I took from that is this: Nature "knows" which colors will "go". Think of the green of trees against the clear blue sky. Here in California, at this time of year, the orange of the California Poppy against the green grass on the hills and along the roads, intermingled with patches of blue and magenta lupine, all with a backdrop of a blue sky is the definition of "eye candy".
There is the issue of value and hue, but to enjoy a real confidence-builder, just go outside and look around and key on some the magnificient color combinations in the landscape . Then, apply that vision to your fabric selections. I guarantee that 99.9% of the time you will be able to stand back, look at your work and smile.
Oh, one trick that does help with "mooshing" the various colors of the fabric together is to stand back, squint, and "see" what looks good to you. Or, you could find one of those door "peep hole" inserts (available @ the hardware store), and look thorough it...same result as squinting. And lastly, my favorite for those of us who are near-sighted, take off the glasses (or remove the contacts), step back and look at your selections.
Whatever you do, go look at the landscape around you for some color combinations that just POP!