Honestly, your quilts will turn out best if you just buy fabrics you LIKE in combination.
The really important part of choosing fabrics is CONTRAST. You want each fabric to show and each piece of the quilt block to be distinct. Sometimes you want bold contrast and sometimes you want a softer look. It's fun to play with the Electric Quilt software to get an idea of how things look in different combinations and fabric placements. We used to have to do that all on graph paper with colored pencils! :lol: