Poynter.org: Color Contrast & Dimension in News Design is an interactive explanation of color with examples and excercises.
Joe Gillespie’s site on design topics, tutorials, tools, and the Web in general. He has some nice pixel fonts, too. Updated monthly. wpdfd.com.
Palette Man has an interesting interface for trying out various color schemes. Students can use it to find a set of four colors and see how they like the combination. All Web safe colors. Color Schemer has a color tool for sale and an online version that supplies you with a set of compatible colors for any web safe color you pick. Very nice tool. An online color site somewhat similar to Pallette Man is Web Color Visualizer which allows you to change background and text colors to test how they look.