I use the 'background color'. If the background is blue, it goes with the blues, white or off-white goes with the whites, etc. This works for me generally, until the other day when I was looking for a specific fabric on the 'green' shelf, and the background was actually off-white. I don't think there is a 'perfect' system, just one that works most of the time. I do have a separate shelf for a project that is still in construction so I don't accidentally use an already designated fabric for a 'quick project'. Holiday specific fabrics get categorized for 'fall' or "Christmas" on a separate shelf or bin.