Hue for Quantities

The widespread use of hue or color to represent quantities in graphs (e.g. blue for 10-20%, green for 21-30% etc.) is a habit that needs restraining. Don Norman* provides a superb explanation for why hue should not be used for displaying quantities:
“… hue is a substitutive representation, and the values of interest are usually additive scales. Hence hue is inappropriate for this purpose. The use of hue often leads to interpretive difficulties. Many colorful scientific graphics, usually generated by a computer, use different hues to represent numerical values. These graphics force the viewer to continually refer to the legend for mapping between the additive scale of interest and the hues. Density, saturation, or brightness would provide a superior representation.”

*More on graphical design principles can be found in Things That Make Us Smart (page 71) by Donald A. Norman. Cambridge, MA: Perseus Books, 1993