it seems it may actually be the case the some people perceive an inverted spectrum. ie the cones in their eyes responsible for triggering a perception of red in the brain actually creates a sensation of blue, and the blue cones create a perception of red. Has to do with the pigments in the cones being reversed...

So they perceive "red" differently, although they still understand it in context and if you asked them what color the stop sign was they'd say red of course...

Now you know, and knowing is half the battle!