You're confused how pigments work. You are thinking mixing of pigments is an addition processes when in fact it is a subtraction process. You are taking away color when mixing pigments, especially when trying to create black. This is because you are absorbing even more light as you add the pigments together. You are thinking you are simply adding in color which isn't true.
Some artists didn't know physics and the classical artists didn't have such information available. So the mixing of colors was poorly understood for a long time and continues to be poorly understood. Any artist doing work on a computer does need to learn how light emission actually works, as the monitor works by light emission. When it comes time to mix layers it is a good idea to understand how all of this works. For this reason most artists now of days have to actually learn how light and hence color works. There is no difference between light and color.
You need to remember color is an emission or reflection of light. In the case of a lightsaber you get the colors due to a particular spectrum emission. The more crystals you add, the whiter the blade becomes. The only way to have a light saber be black, ie no light emitted, is to turn it off.