Yes there is dirt and grass, if you go offroading you will notice how little grip you will have when you come back onto the track, and turning with dirty tires will make you slow down much more than usual or else you will go off the track and get your tires dirty again.
It doesn't really matter if your tires are not at optimum temperature, if they are still blue yes you'd want them to get a green tint, but I would choose bright blue over red in this case any time.
When your tires get overheated and turn red in the F9 display, it becomes very hard to control the car, much harder so than the effects of dirt.
Get out a RWD car and put it up against a wall and do a burnout until the rear tires turn red in the F9 display and try and drive it.
No need to ask when you can do.