[GUIDE] What is Selective Coloring and How does it Work?

WARNING: IMAGE HEAVY
[part one]
[part two]

So many of us use selective coloring, but do we even understand what it IS? Do most of us even really know HOW or WHY selective coloring works the way it does? Well here I am, trying my best to explain in case anyone is interested in learning how to use the popular tool better :]