In your car, you'll need a "when green flag clicked" block from the "Events" section. You'll also need a "forever" loop and a "if _ then _ else _" block, both from the "control" section.
Use a "sensing" block called "touching color ___" and put it in the "if" blank. Use the eyedropper tool to select the exact color of your grass. Be sure to watch the gif if you're unsure of how to use the eyedropper tool!