Converting knowledge to painting

Well, this is the part you've been waiting for. Sorry, but you're gonna have to do a lot of work yourself from hereon out.

Understanding something means that you can apply it practically. Understanding addition means you can calculate 3+3 without the help of a computer or calculator, understanding what end of the brush to use for painting means you won't stab your subject with a handle etc.

Understanding colour theory means you'll look at your given subject, you'll choose a colour and then reflect upon how it will interact with other colours, lightings and shapes. 

This is something that comes with practice, in the beginning it might be helpful with a cheat-sheet, a colour wheel, or similar. After some time you'll remember these things and not look at a colour wheel as often, and when you're really good you'll know these things by heart. Instinctively almost.

So next time you pick up a paintbrush, try thinking an extra round before applying paint. :)