I am trying to create the map for the current D&D campaign I am in (Already got the OK from the DM) but, I was wondering how do i go about coloring the map correctly?

Like i don't just want the grasslands and such to just be a solid color. Note I am very new at mapping, I have GIMP. and have been looking for tutorials to help with not only mapping itself but coloring of the maps and such.. If anyone can help point me in the right directions i'd be very appreciative cause i've been looking for like 2 months with no good results.