As graphic designers retouching photography is essential skill You need to master. You could talk about principles, but at least I started with simple photo editing on Photoshop, trying to take off red eyes, lumps and change backgrounds of pictures I was using. In this You’ll find everything You need – facial and portrait, teeth, lips, eye retouching, background remove as well as simple color adjustments in video and image-based tutorials In fact for color adjustments I would suggest You look for some premade actions available on deviantart, very handy if You’re just starting out. This list is not so big because it is carefully picked, I am starting to think it’s better to get quality than quantity. Enjoy amazing, retouching and advance photoshop tutorials.