Hey there, Since old times it's known that white shirts, tainted by blue, will look brighter, thus whiter. I heard that it has to do with bipolar cells (Wh, Y B and R G) responses. If we put them in 3D plane, then vector's module is longer if both Wh and Y B cells react as opposed to reaction of only Wh cell (Pythagorean theorem). But why exactly we have to use blue to get brighter shirts? Red colour would also make vector's module longer.