Intrigued by Genga/Douga writing

On many of the production sketches there is red pencil writing- usually numbers. I'm not referring to the black letters/numbers in the top right corner but notes sketched within the frame of the page.
It may seem a silly question but I wondered if anyone knew exactly what this infomation conveys?

I know that the little 'x' marks are visual guides for the artists to denote empty space and I thought the numbers corresponded to a paint colour chart, but I'm not 100% sure.

Some dougas, usually the initial Key ones, can be littered with numbers and arrows but I've yet to find a pattern to define when and where they're marked in a sequence.

I just figured they were color numbers.

paint by number?