Color Schemes for Choropleth Maps
"Color is clearly worth the extra effort and expense it adds to map making because it permits greater accuracy in map reading. In addition, people prefer color maps over monochrome maps." (Brewer, 1997).
The diagram below illustrates a variety of spectral schemes used in choropleth maps. Notice how the colors "ramp" from lighter to darker shades. Use this diagram as a "guide" in the development of a color scheme for your map.
Source: "Mapping Mortality: Evaluating Color Schemes for Choropleth Maps", Cyntian A. Brewer, Alan M. MacEachren, Linda Pickle and Douglas Hermann, Annals of the AAG 87(3), 1997, pp 411-438.