Go to the Hardware Store paint section, look around for remnants or throw aways... these are basically botched paint colors. The odds of you finding white, cream and beige are VERY good
; lets face it they get used a lot on houses. Odds are you'll be overbuying the volume you need but you'll be able to paint up any terrain you build to match quickly and I'm sure you're aware of all the advantages of using latex for terrain.
You can mix whatever you find together in varying amounts to get the desired effect as well as some tints and shades for drybrushing. You really can't go wrong, whatever colors you end up with will mix together to form some sort of chromatic grey.
If you're not feeling adventurous you can have a quart mixed to be your base color...
When you do Static Grass be sure you don't apply it evenly. Sponge on the glue if possible so you get some nice natural shapes and patterns, and just as if you were painting be sure to rotate the sponge once in a while.