AS 3 race game, need help

Hello, i’m a belgian student.
For school I have to make an assignment, make a site with an easy as 3 flash game.
I was following this tutorial.

link :
www.emanueleferonato.com/2010/06/16/create-a-flash-racing-game-tutorial-as3-version/

You can download the source code from the site.
But the problem is, i want to change the color of the ‘ground_mc’
Now it is white and if i change it to another color the car moves like it is in a ‘slow’ zone. So i can’t change the color.
Can somebody help me where in the code i can change this?

I really need help,
thanks.