Just returned from my local Sherwin-Williams, where I found a new gizmo on the counter.
Called "Mobile Match," it's a pocket-sixed thing that you press against a surface ... and, a moment later, it tells you what S-W color matches the surface. Cost is under $60.
While I find some internet evidence that this thing has been around at least 1-1/2 years, only this week did it reach my area.
It's not perfect, and color chips are not always available for comparison. Any surface with multiple colors - wood grain, bare masonry, etc. - can be tricky. You'll need to take multiple readings and compare multiple chips to be certain of your 'best' match.
Look through the FHB "Great Moments" series, and you'll even find a story about the homeowner who learned the hard way that there was 'purple' in his stone house :D