The answer is in your question my friend.
Line drawings, you need to draw a little more and fill in the gaps with colours of your choice, example image attached.
Although I can not help you with the drawing bits, thats for you to do.
If you open up the library you will find the graphics in there, you will be able to edit each one in turn, yes, the blue can be seen through the hub cap as i did not fill in the gaps ( just quick example for you) thats for you to do, just make a back up copy first, then mess around with the copy.
As for the hub caps, fill the gap in with a silver/grey maybe!!!