I use undiluted tempera with a brush to paint th ground...It'll dry super-fast and dead-flat, and won't soften your dried celluclay. I also use a good-sized dollop of it in the celluclay as I'm mixing it... Big ol' blob of brown will keep the gray from turning up in a place you can't quite reach to paint..The thing about tempera to remember is that it will reconstitute with water or other liquids, even if it's been dry for weeks... It's probably a good idea to seal it with a clear coat of enamel. The stuff I use (Fresco) is about 1.70 a pint pre-mixed, but you can also get powdered tempera, but it's considerably more expensive... But all you need on hand is the primary colors (red, blue, yellow), plus black & white... All the colors you'll need can be mixed from those five...
I don't use model paints to paint groundwork though... They're too expensive for that kinda work..