Up until now, there have been no explanation of Graphical Locater map colors, but there have been very few questions on this matter. I hope this means that the colors are "logical choices", which are readily interpreted by most people. Almost all of the 10,000+ maps in the Graphical Locater project use the same basic colors. A few of the custom maps, naturally have custom colors, but most of the main features follow a similar color scheme.
The brown-scale shaded relief was developed to give the correct impression of hills and valleys, while avoiding the lunar landscape impression that a gray-scale gives. The reduced number of colors (11) helps minimize file size. The road colors were adjusted to allow those with the most common color vision deficiency to make them out better.
Some color related links: