2010-03-15 00:00:00 -0400
Smashing Magazine has a 3-part introduction to color theory that covers the basics fairly well: part one, part two, part three.
NASA has done quite a bit of research into use of color when conveying information.
Multicolor is a tool for searching for "interesting" flickr images that embody a particular color.
It is possible to generate color pallets from a picture using several websites.
COLOURlovers and kuler are communities of designers who create and share color pallets.
When creating color variations often what is wanted is a change in value. Value is the lightening or darkening of the same hue and saturation of a color. I use this kind of variation when deriving appropriate borders for buttons based on the primary fill color i.e. lighter borders on the top and left, darker borders on the right and bottom. I do this with a color picker that can switch between RGB and HSV representations of color. Enter the RGB color encoding, switch to HSV and adjust the value slider, switch back to RGB to get the new hex values for the color.
There are websites that have this functionality but most bitmap editing programs have this capability in their color pickers. The GIMP, Photoshop, and the Mac OS color picker do. I am sure there are others.