Marcelo Gattass has a very useful page ( Color Conversion Library in ANSI C ) that provides straightforward source code for color conversions.
His site has made it much, much easier to make progress with my recent Visio activities: Automating Visio 2007 with AutoVisio - now on CodePlex
Things to note about his site:
If you need to do color conversions, I suggest starting with his page.
Other Resources
EasyRGB.com