Different runway surfaces gives you different resistance, and you need more runway  to take off in grass than asphalt for example.

Is this modeled in Prepar 3d or is just a different color runway (green for grass , brown for dirt and so on).

 

This shouldnt be difficult to model.

Yes and no, since there is little information available on P3d with regard to your question. As to FSX, wet, snowy and icy runways have a lower coefficient of friction. Dirt and grass runways have a higher coefficient of friction. But are they modeled correctly? That's really another question. There is an add on for FSX that supposedly "fixes" the problems:

 

http://secure.simmarket.com/fsps-frictionality-fsx.phtml

 

My guess is that P3d is similar to FSX with regard to runway friction, but only someone with  intimate knowledge of both the P3d SDK and the internal workings of the sim would be able to answer your question definitively. Try posting your question on the official LM message board.