In Minecraft, biomes are (supposedly) decided by an area's calculated temperature and moisture. A very "dry" area may be a desert, a "cold" area may be a tundra, etc.
This also affects things like grass color. In my experience, forests tend to have darker grass than plains.
If I, for example, plant a lot of trees in a plain, will it be handled as a forest and make the grass darker? (There's a specific area of my map that would be great for building a base, but the grass annoys me.)