It is primarily a tool for checking foreground & background colour combinations to determine if they provide good colour visibility. It also contains functionality to create simulations of certain visual conditions such as colour blindness.

Determining "colour visibility" is based on the Contrast Ratio algorithm, suggested by the World Wide Web Consortium (W3C):

The Contrast Analyser was developed by JUn of the Web Accessibility Tools Consortium (wat-c) in collaboration with Steve Faulkner of TPG (Europe) and wat-c.
MAC version developed by Cedric Trevisan of TPG (Europe) and wat-c.