Just Paint

 Here is a great resource for painting/color mixing, including a downloadable PDF:

http://justpaint.org

Mastering Color by Richard Robinson

I recently stumbled across Mastering Color by Richard Robinson in New Zealand, which is a great video painting course for any painting medium - oils, acrylic, watercolor etc - it's all about how to see color properly and translate it to canvas. It covers lots of in-depth material in a way that's really easy to understand. I totally recommend this for all painters who need better control of their color - and lets face it, most of us do! Here's the link to the website:

http://www.livepaintinglessons.com/colour.php

Oh, here's the chapter list too:

1. Color Theory
2. Seeing Color
3. Describing Color
4. Value
5. Mixing Color
6. Manipulating Color
7. Color Harmony
8. Light Effects

You can watch the first chapter for free on the site:

http://www.livepaintinglessons.com/colour.php