Start off with a monochrome signal, RGB is simply three monochrome signals, one for each colour - composite is a an encoded form of RGB, essentially a B&W high resolution image (so it's compatible with B&W TV), with a very low resolution colour image smeared on top.
Think of a nice high quality B&W drawing, then give it to your kids along with some wax crayons, and allow them to crayon over the image - that's what a composite image is like.
But start with B&W first, it all stems from there.