Most of the time its primarily impulse. Color is usually what I notice first then I go for how it feels and the price. If the price is out of line with the quality of the fabric then it will probably not go home with me.
Sometimes I'm trying to find one more fabric to go with 2
or 3 I already have but need another to complete the design I have in mind.
A few weeks ago I had been going through my stash and realized that I had lots of some colors and not many of others that I really liked so I looked particularly for teals and aquas on my next shopping trip.