Hyperphysics (C.R. Nave, Georgia University): Short pages on various topics related to colour, light and vison, set out in web arrangement
http://hyperphysics.phy-astr.gsu.edu/hbase/ligcon.html (Light and Vision)
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colviscon.html (Colour Vision)
Nassau, Kurt. The Fifteen Causes of Color.
http://flash.lakeheadu.ca/~mwesner/COLORreadings/Color,%20Light%20&%20Interact/ (13 jpg/pdf pages)
Longair, Malcolm. Light and Colour.
http://flash.lakeheadu.ca/~mwesner/COLORreadings/Ch3-Light&Colour/
Causes of Colour (Webexhibits)
http://webexhibits.org/causesofcolor/
Brown University: Series of applets for demonstrating colour theory, such as the interaction of spectral distribution and reflectance curves on reflected light.
http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html (Online/Windows/Mac)
efg2 Computer Lab Color Projects: Various free programs.
http://www.efg2.com/Lab/Graphics/Colors/index.html (Windows)