Colours.

Title: Colour Theory
Notes: Explains the mechanics behind working with colours, and how to get colours that go well together. Very useful and informative.
 
Title: Symbolism of Colour – Using Colour for Meaning
Notes: Extensive list of links describing the meanings often associated with different colours.
 
Title: Colour List
Notes: A huge colour table of over 2000 colours, their RGB, hexadecimal designations, and variations on gamma correction. Good for knowing exactly what colour you need.
 
Title: Yafla Colour
Notes: A wonderful colour matching utility, for selecting colours that go well together.
 
Title: ColourMatch 5k
Notes: Simple utility for selecting colours that, well, match.
 
Title: Colour Blender
Notes: Another great colour-matching utility. Select colours and it will create a spectrum of colours that will flow well together.