Cheap, inexpensive, and looks pretty dang good!
Step 1: Materials, Etc.
What I'm gonna do is just spray paint my desired colors, and try and make it look as realistic as I can. I'm using a dark green felt so I'm using White, Light Brown, and Light Green. I don't want to overload it with light colors though so I'm also using Dark Brown. Now I also know that in a lot of Gaming Books on how to build maps, the books say to use Red, to bring out the Green. Of course you may use whatever colors you like. It would still look goood with just Light Brown or just Light Green. Whatever you wish.
Step 2: When That's Done.
Ok so your done painting and it's looking good. You can keep it like it is and it would be perfectly fine but I'm gonna take it one step further.
Step 3: Ruffing It Up.
Your mat will look a lot better if you can get the fibers in the felt to stand up and look like grass. I grabbed an old wire brush and I'm just gently brushing it along and standing up the fibers, not to hard to pull it out but just enough to get it to stand up on it's own. Of course some of the felt will come out onto your brush but that's good cause you may use it for flocking, bushes, etc. Have fun gaming and may many victories be yours... lol.