To arrive at a completed auto portrait in colored pencil isn't much different than an oil or acrylic painting. I begin with a search for good reference art of the subject to illustrate.
Then I begin the black and white pencil drawing process to get the proper perspective angles of the vehicles and their surroundings. Once I have the line art just the way I want it, I transfer this art to an illustration board.
To arrive at the correct color for each vehicle, I refer to auto paint color charts and then match them to colored pencils (sometimes it takes two to five different pencils blended together) to get the color just right.