In this tutorial you will explore the Difference layer blend mode and basic color theory. Before computers there were two basic color theories. There was Light Theory and Pigment Theory. With the advent of computer monitors, there is now a convergence of the two theories.
Lets take a look at the core concepts of each theory.
In light theory, you add colors to increase light. If you take three light-projectors one with a red filter, one with a yellow, one with a blue filter and shine them on the same screen, the screen will become near white. What is happening is that as you shine each light projector on the screen, there will be an increasing amount of light in which each projector's filter is supplying the components of white light which the others removed. Light Theory is considered Additive, in that you add colors to increase the lightness. In the above example I use red, yellow and blue.