What two colors mixed, say like in the Testors enamel line, make a good muddy road look? and/or for a dirt road look?
I dont mix colors. I shop till I drop for colors though.
For a muddy road… first I paint my darkest colors, then highlight with lighter colors. You can use gloss paints or just seal it with a gloss clear coat at the end. Try dark browns, tans, blacks, greys, buy a bunch and just experiment because they look different when they dry [banghead]
good luck 
for dirt roads i use a 2-1 mixture of dirt and white glue. it makes a paste then i spread it out on my dissired surface and use a custom made tank tred half to detail the surface
I used a mixture of burnt umber, earth brown, and leather in my Andersonville dio by the wagon.
I also used white glue to thicken it as duckman suggests.
MF