This was the first car what I sketched up after the  class,

I sketched the car with black ballpoint pen and started to  color it  after that....which now I know it s not the best idea,because the marker make the pen ink bleed and turn them to purple!

So what I would recommend is sketch with a light grey marker, which almost invisible on the paper , so with that you can get the basic proportion of the car and then you can add thin lines with a pen which is marker proof and then you can  add the extra colors and shadow etc.

or you can sketch with ballpoint pen and when you ready with the line work you just scan it and print it out and color the copied version, this way you have more room to  experiment :)

Here below I have printed out one of my line work sketch but made it really light, so after that I just used markers to  color it, and I haven't used pen just a  0.05 liner on a few spot, the result became one of my best so far.