I almost always start with fabric. (My fabric shopping is entirely dependent on my mood. Any success I have finding great fabric at a great price is entirely dependent on luck.) If I see a pattern/quilt that I really want to make, I mentally make it with fabrics I have in my stash. If I have most of the fabrics I need (and enough yardage) and they truly do look good in that pattern, then I usually need to make another trip (with my envelope of fabric snippets) to the fabric store for whatever pieces/colors I'm lacking.
However, I have on occasion had an attack of "I MUST MAKE A QUILT LIKE THAT!!!!!" and have had nothing in my stash that was right, so had to buy all the fabrics to make it.