Crayons are pretty transparent stuff.... and they don't build up well at all. The first layer holds fast to the paper and subsequent layers don't want to go on. Crayons characteristics translate pretty closely to oil pastels too... Below is a drawing I did of my duplex leading out to the alley (I have an alley!!), but I did it at night! in the dark! It was pretty interesting... It's incredibly hard to tell what color's you're using beyond hints of the cast, so you end up relying on value a lot more. Below that is a gouache painting from one weekend afternoon. They were each about an hour.