To be truly gray, that is totally desaturated, all values in the RGB "formula" are equal. R0, G0, B0 is black. R255, G255, B255 is white. R128, G128, B128 is 50% gray. However, R 27, G 12, B 27 is not pure gray. It has slightly less green in it, so it will be a tinted gray. In order for it to be a true gray it has to have equal levels of all three color guns. And because the color gun values only go from 0 (full off) to 255 (full on), the most shades of gray you can get are 256. As a result, there are only 256 possible shades of gray in any image, regardless of its resolution.