In 2004 Stephen Few wrote a book called Show Me the Numbers: Designing Tables and Graphs to Enlighten. In a 13 page paper on Practical rules for using color in charts he discusses and illustrates nine rules of graphics, which are:
Rule 1: If you want different objects of the same color in a table or graph to look the same, make sure that the background (the color that surrounds them) is consistent. (A gradient background just adds confusion).
Rule 2: If you want objects in a table or graph to be easily seen, use a background color that contrasts sufficiently with the object.
Rule 3: Use color only when needed to serve a particular communication goal.
Rule 4: Use different colors only when they correspond to differences of meaning in the data. (Adding a different color for each country to a bar chart adds nothing when you already have identified their names).
Rule 5: Use soft, natural colors to display most information, and bright colors and/or dark colors to highlight information that requires greater attention. (He gives an example of palette with eight soft natural and bright highlight colors).
Rule 6: When using color to encode a sequential range of quantitative values, stick with a single hue (or a small set of closely related hues) and vary intensity from pale colors for low values to increasingly darker and brighter colors for high values.
Rule 7: Non-data components of tables and graphs should be displayed just visibly enough to perform their role, but not more so, for excessive salience could cause them to distract attention from the data. (The scales and borders should not visually overwhelm your data).
Rule 8: To guarantee that most people who are colorblind can distinguish groups of data that are color coded, avoid using a combination of red and green in the same display. (About 10% of men cannot tell red from green, and identify traffic lights by position only!)
Rule 9: Avoid using visual effects in graphs. (A plain bar chart is preferable to one with three-dimensional rods).