An example of a poor colour scheme (default rainbow scheme in IDL, top) and an optimal colour scheme (sunset colour scheme by Paul Tol, below) is shown below, including rationale why the sunset scheme works better:

Most rainbow schemes contain bands of almost constant hue with sharp transitions in-between, which are perceived as jumps in the data. In this example large areas are green without features, while for example the yellow line in northern Australia implies a sudden change that does not exist in reality. The oceanic trench at top right can be seen over the whole length in the top figure, while it becomes difficult to see near Japan in the bottom figure. This shows that it is more important that a scheme is smooth than that it contains many colours. In addition, colour-blind people have difficulty distinguishing some colours of the rainbow. In the bottom figure, the yellow spot in the middle of the Sahara is not visible in red-blind vision, making the green areas effectively even larger than they are in normal vision. Last but not least, the sunset scheme emphasizes more clearly the average (light colours) and the extremes (dark colours of contrasting hues).