I have built a diorama in a pine box out of plaster. I have shaped the terrain to my liking. Now I am hit with a bigger problem. How do I make realistic textures (and colors for that matter) for pretty dry (not quite dirt) mud and grass?

The easiest way to add natural textue is to use the real thing. Fine sand (pet shops) or sifted dust shaken over the area will give a realistic texture. Paint on some white glue diluted with water tothe area, shake on some dust and leave until dry. Add small stones at this stage also, either from your garden or cat litter. When alls dry blow off the surface and you should be left with a nice texture. For grass, static grass is hard to beat. Fix using diluted white glue again. Leave for one hour, then blowing will rmove any grass not fixed and make the grass stand. For longer grass use brush hairs. Paint with colours that look right to you.
Visit your localhobby shop or railroad hobby shop for better ideas and these items. Look through the threads on the diorama forum andyou will find lots of descriptions about these, andin ore detail. Good luck!