i see. it's possible. i won't do it though ^^
in fact it should be pretty much simple - there are two ways:
1) turn on lightmaps in client - afair all ground will be shown as pure white regardless of anything. to get desired color you'll just need to somehow modify the lightmap entry in gnd.
2) (more easy way in my opinion) modify texture in .grf in a way that makes image single-colored. you won't be able the exact desired color this way though since every map has a global color defined which somehow mixes with the one on the texture. for example, if texture is pure white you will get ground color like RGB(124,124,148).
p.s. still i think you are going little overkill - there are all sprites available, so you can freely compound them as you want in any way. it's up to you, though )