In GRASS to define the colors of a raster you normally go to the menu Raster → Manage colors → Color tables. Alternatively type r.colors on the command line. Both will open a screen where you can select the raster layer and one of the many pre-defined color tables. There are a number of additional options, most are self-explanatory, otherwise check out the help function.
Instead of a pre-defined color table, you can also use a rules file Continue reading