Five Stops From The Edge by Bob Radcliff
Almost a "Zone System" in a color.
Colour Theory as Applied to Landscape Photography by Michael Reichmann
Good description of first and second order colors, complementary and harmonizing colours, intensity of light. Really nice photos as examples.
Composition: Understanding it - Using it by Larry Seiler
This atrticle was written for painters, but these image balance rules are perfect for photography.