You really will learn best by doing, and learn to trust your gut. Sometimes the color is right, but the scale is off, or vice versa. For checking contrast between dark, light, and mediums so that your design stands out, take a picture
in black and white. The contrast will be easy to see without color interfering.
I generally choose a focus fabric then coordinating prints. I try to make sure I vary the scale, have some tone-on-tone or solids to help the eye rest, and then I fan them all out on the bed. If one just doesn't "feel" right, I remove it. THen a more settled feeling comes over me. I do the picture/"feel"/remove or replace routine until the settled feeling turns absolutely happy. Hard to explain because of the intangible quality of art and aesthetics, but I don't choose my fabrics based on a number unless the pattern is specific. I prefer to use colors in a range rather than a whole lot of the same fabric. And I have no patience for making the same block over and over with the very same fabrics, even though I love the result.