I always 'group" my fabrics with a light, medium and dark... and buy all that is left on the bolt up to 6 or 7 yards of the light, less of the medium and lesser of the dark. If I cannot match it, I leave it there.
Or if matching fabric is in the same line, I might go for more if I can "see" a possible project: Irish treble chain.
I might add to my stash if it is on sale and I am short on "browns" or "beige" or "white" or "black"; it is never short on blues or bright colors. Or I can just make up excuses and just go for more fabric....