Recommendable usage of colors

• Emphasis (particularly when used as background)
• Grouping or separating neighboring objects
  (particularly when used as background) 
• Coding of discrete data
(not more than 7+/-2 colors if codes have to be remembered)
• Window systems: marking the type of window or application
• Graphics: Visual separation of overlapping graphical elements or labels
• Naturalness in photorealistic images
• Warnings, status reports
• Depth in 3-D graphics
(reddish colors appear closer, blueish colors more distant)
• Coding of continuous data in information landscapes
(e.g., false color representation)
• To increase the attractiveness of an interface, following guidelines for color selection (increased attractiveness does not necessarily imply an increase in usability)