- Avoiding issues with color blindness
- Help picking color palettes (principally for thematic maps)
- The value of subtly colored base maps for map mashups
- The importance of choosing 'natural' colors for symbols (water = blue not red)
In researching my currently running course on communicating with web maps I came across some other best practices, mostly via the chapter on color in Muki Haklay's book.
Background = Blue: We evolved doing things like picking berries from bushes in Africa, its therefore not surprising that the natural background color of that African landscape, the blue of the sky, is one as one that we still naturally associate with background. Note how the red water above comes forward compared to the blue version and swamps out the map detail around it (map courtesy OSM)
Color Wheel Opposites: The color wheel is a representation of 'color space' with hue around the edge. When choosing colors for symbols (such as placemarks) that need to visually stand out from each other, choose colors opposite each other on the color wheel. A well used variation of this is yellow and blue.