If you knew the RGB values of two different pixels, how would you decide
whether they are the same "color" or not? I was looking at color
swatches yesterday: the place I was had a relatively small collection,
with only about 28 different shades of "red", some of which were the
same hue but had different glossiness. In larger stores, I've seen over
100 different shades of "red" -- and that's not even counting the pinks.