Light Grey, or whatever color is above Magenta. Also, you don't have to post an entirely new scene every time another marble is taken. You can just update the scene by clicking the Re-Share button. If you do that, then we won't have 5 of your scenes cluttering everything up. Not saying you have to do this, but it's reccomeneded.
The only thing I found that you could improve is coloring the car a bit, as well as Mario. But that's only a minor issue. Adding in the +2 course bonus, your score is 10/10.
I have to apologize to Yoshi, as he did the challenge and I never looked at it. He got a 3/10. So, even with his bonus, he would have not received a prize, as it would be a 6/10. So, even though it's not a big deal, I just figured I'd apologize.