Although you've probably come to this conclusion yourself, there's no point in buying any green "grass" for 1/48th-1/35th and larger scales... Those colors are for HO and N scale railroads and the like, and are just too garish and horrible in larger scales... Stick with the tans & browns in the future since it's easier to make stuff greener than it is to "ungreen" it... I'm not even sure that those shades of green that Woodland Scenics makes are even found in nature, lol...
As for the types of paints to use, that's going to depend on your personal taste, but I use the 1.29 rattle-cans from Wally World to paint the grass in general, braod, sweeping motions and add discolorations, tracks, trails and such with tempera paint from an airbrush... Tempera is my favorite "ground" painting medium, since it's easily thinned with water and a couple drops of dish soap for a wetting agent. The stuff is also available in arts & craft stores for about 1.77 a pint and a little goes a long way... You won't find any ready-made colors like in craft paint (I do color my celluclay with pre-mixed brown tempera though), but with a set of the basic primary colors (Red, Blue, & Yellow) and a bottle each of black & white, any secondary and tertiary shades you can think of are easily mixed... I prefer the pre-mix paints, they're about the consistancy of pudding, but the powdered tempera has a number of uses as well, especially when it comes to dry-blending between color bands on grass & earth, and even work well (although ya gotta be careful) as weathering powders... They also can be reconstitued with water even after they're dry...
If you have a Hobby Lobby or Michael's nearby, the brand I use is Fresco, which goes on sale every few weeks for about 1.19 a pint. They also come in small bottles, 4-oz. or so, in "six-packs"...