Color check in bitmap

Is there any way to check what kind of color there is in a sertain pixel in a sertain cordinat on a bmp-texture.

I am making a simple cargame in 3D inviroment and the road is black and the grass is green. If i dont ride on black the car will go slower, (i know how to control the car).

I would be sooo happy if someone gave me a quick answer, mayby including code exampel.

look up glGetTexImage(…)
as with most get things it mightn’t be the quickest thing performance wise

Thx Zed, i will check it up.

Iread in another post about glReadPixels, how does this work?