I pick a quilt pattern first and go from there. Many patterns call for a focus fabric that has multiple colors in it and you then choose other prints that have those same colors in them and have those be different values, as well. Other times, I get an idea in my head of what I want in the end and choose colors that go together or contrast, depending on the design. Sometimes, I've not been able to find the exact color fabric I wanted, so ended up using lots of thread to achieve the look I wanted (when fmq). You can always go with analogous colors, complimentary =opposite each other on the color wheel, etc. like others have mentioned. I personally think you can't go wrong with using whatever colors you like together. I do think it's a good idea to cut out your fabric choices and place them on a pattern of your block to preview what they'll look like - before you go to all the work of sewing a block together. It's much easier to change out a small piece of fabric from a block when it's glued on verses all sewn together.