The colours mess up, because the non-recolourable bits (dash, etc) are within the same object as the recolourable stuff; you need to keep those things in separate objects. Select all the grey, white, orange, red, etc bits and separate them by pressing P.
Also, I noticed the steering wheel is not working properly, it should go from -90 degrees to +90 degrees. Yours currently goes from -90 to 0 degrees. The difference between the two states should be 180 degrees, not 90, though I might have not been to clear on that.
Also, your car is very prone to roll-overs which means the mass centre is too high in the model. You can fix that in the script or in the model. If you want to fix it in the model, all you have to do is to move the body upwards in Edit mode until the bottommost vertices are more or less in-line with the object centre (pink dot).