"Brightness? Hue? Saturation? Then segnemt according to that property, Just taking the 3-D RGB vector distance isn't the greatest idea, because humans don't differentiate colors according to that metric."

This concurs with what others said, so we are all pointing to the same direction that, RGB is not what human being used to find out edges. This is actually sort of AI, as part of it is to understand human behavior, and then think of how to repeat/simulate it.