I have observed the following phenomenon: When I shine coloured light (say red) on a surface that has its complementary colour (in this case green) it becomes significantly less saturated (even to the point of becoming completely desaturated, i.e. grey). I had no opportunity to try this out in darkness, but my intuition is that a perfectly green surface completely absorbs red light and it would appear as if there was no light at all, i.e. remain black. This seems in conflict with what I have learnt about light so far. There are two systems, a subtractive and an additive. The subtractive is when the sum of all colours gives black (for example with paints) because it subtracts certain wavelengths from being reflected. The additive is when all colours give white, because they add up to the full spectrum (which is white, e.g. light). Shining a coloured light onto a coloured surface seems to fall under the subtractive category. But is it not the case that when you look at the wavelength histogram of a coloured light it is not a column on one very specific wavelength but rather a bell shape around it? If this is true, subtraction could work only with two identical curves. But with red and green the curves are different and subtracting them would create leftovers on the outside edges of the curves which would produce a colour. Where am I wrong? Could someone please explain and/or proof this to me. References and further reading is appreciated. I do not fear math.