One YouTube channel that I keep my eye one is the one created by the guys at Level Up. Ok, these videos are hangouts, and may not be the best edited and produced films out there and they are longer than the usual YouTube video (over 2 hours most of the time). However, what I really like about these videos is that the organisers interview and talk with guest speakers who let you into their Photoshop Digital Painting world, and they talk about their workflow and processes. I think this is a great way to learn about other processes that Photoshop does, as well as some great nugets of information that only Digital Painters know. There are lots of them to watch and are great to have on just in the background, enjoy the shows here.
Here is one to get you started, Colour Theory