"outside jelly" refers to a vehicle having a candy colored paint job. The vehicles candy paint exterior has a slight resemblance to the coloration of jelly/jam (preserves).
"inside peanut butter outside jelly" refers to having a tan/brown leather or plush interior in your vehicle, with a (metallic)candy paint job on the exterior. (These color schemes are commonly used on Lowriders or American Muscle Cars, such as Cheverolets.)
