[Update: there is a nice working plugin that does the trick, see here]
In GIMP exporting multiple layers to one pdf file is surprisingly difficult. I found two solution, both require additional software besides GIMP, but are fairly easy to implement. Continue reading Exporting layers in GIMP file to single pdf file
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 Color tables in GRASS GIS – using the command line