It might be useful to look at a color wheel of web safe colours. You could use the colour values from opposite sides of the wheel to display text.

There is a rather nice colour wheel on this website: http://www.visibone.com/colorlab/big.html. The wheel is interactive and demonstrates both the luminosity aspect (choosing black or white text) and the use of opposing colours in the table on the right hand side.

A nice question.

inman