The careful use of colour – of hue, saturation and lightness – can improve a user interface greatly.
Sometimes the best use is to avoid distraction, allowing people to concentrate on differences that matter; sometimes the best use is to add information, revealing differences that were previously hidden.
Mark Miller (of DotNetRocks and DevExpress fame) spends a lot of time focussing on this area, treating it (rightly, in my opinion) as a competitive advantage.