I've been taking an online painting course at Schoolism. Here are five pieces for the first week's assignment of painting under a diffused lighting situation (white light, soft shadows). It's been helpful in getting me to tackle difficult lighting situations and just getting out there and practicing more. I'd recommend it to anyone interested in the animation industry, as most of the courses on Schoolism are geared towards technical skills. There's also a really cool option of taking a version of the class where you can get video feedback and crits from the professors themselves, but those are much more expensive and spots fill up really quickly.