The colour tint should be subtle. IF you were using traditional media, I'd suggest mixing in some complimentary browns, but digital colour is really impersonal >.> I guess if you desaturated the colour. What ever happens though, the deal with light is that with a single sorce, the shadow is always the complimentary of the source at the same saturation. Perhaps you're looking for a warm black rather than a neutral black or cold black if you know what I mean? http://www.graphicdesignforum.com/forum/archive/index.php/t-11201.html
I kept running into "now, if this were oils or watercolor...." . Painter's a wonderful program but digital is just not quite paint. Thanks for the link! I'm going to make note of those formulas.
Some programs are intellegent in the way that they mix like IRL paints do. http://www.artrage.com/ Artrage is a program that is written to SPECIFICALLY replicate traditional media, and apparently it's really close. I don't know cause tablets aren't enjoyable for me to use so I don't do digital art often, but my friend Hannah swears by it. It's cheep to buy the full version of it too in comparison to Photoshop too.