Colored markers are much harder to use than gray scales. Just matching up the tones and shades can be a huge headache. In grayscales, everything is setup for you. You don't have to go out and test the shades or tones. They are all written out for you. I do recommend using grayscales first. Another thing is, you want to start light. Go 10%-40% at first until you are used to seeing the form of a car before diving in fully and using darker tones.