You obviously cannot have a two colors of green in a single XY location in a bmp. Additionally because of the way that mapconv works there is a limit to how close together trees will get before you loose the grass. Does this bother you?
I've put together a Test map: http://www.fileuniverse.com/?p=showitem&ID=1413 It includes the metalmap that was used, for reference.
I've got a couple screenshots showing both 'sparse' folliage w/ grass, about as dense as grassy trees gets, and a shot of the density at which you loose the grass.


and

But please grab the map, perhaps play with it a bit, and let me know your thoughts.
The 'fix' for this would be to use the blue channel for grass, and use shades of green for features. Personally I would trade the grass/tree granularity for having features obviously different (like blue)- but I will gladly defer to the majority opinion if needed.
Took this shot just because I liked it [and I certainly wasn't going for 'pretty' when making this particular map, heh]

P.S. Jello = Yeah
