Probably the most intensely scientific approach to light for painters there's available on the web. It aims a lot of the disucssion at why that scientific system is good to know as a creative, and it's a good if lengthy read. I'm struggling myself, but it's worth it.
Second up is a blog by a rather good painter:
If there's someone that can analyze the decisions master paintings made, this guy can do that. He gives clear demonstrations of why certain techniques in painting work the way they do, and how they are employed in clever and creative ways.
Finally, blog by the Senior Art Director for Wizards of the Coast. Less to do with painting, but everything to do with ideas, and how important they are to a business such as WoTC - and how you go about selling them.