I am not referring to color combinations, there are 16.7 million of those in rgb images. What I am saying is that the maximum difference between black and white is only 255 levels, that is between rgb 0,0,0 and rgb 255,255,255. High dynamic range can represent white as "theoretically," rgb 1,000,000 100,000,000 1,000,000, so that when you display the image on a high dynamic range display or adjust the exposure, the image behaves much more like real life. In the real world, not all "bright" things are the same brightness. hdr allows you to express how bright things are very accurately, while normal 8-bit rgb images cap all bright things at one value, which is pure white.