Is it posssible to implement my own ColorType?

[Osthekake]

I want to generate image files that are limited to 4 colours. (Not shades, or anything. 4 total possible pixel values)
Currently, I do this by having my images as RBG, and carefully making sure to only use my allowed colors. However, due to stuff like anti-aliasing, and sometimes input that isn't properly prepared, I repeatedly have to run a slow and time-consuming algorithm that goes through each pixel and corrects it to the closest match out of the 4 allowed colours.

It would be very helpful to me if this was simply enforced by my framework, and I don't have to think about it any more after I've implemented whatever is needed to create a ColorType.

I doubt this would be very helpful for anyone else, although you never know.

It could be nice if the ColorType could also pack 2 pixels into each byte, but this is not a huge problem for me at the moment, and I can see it causing problems when I want to flip the images 90-degrees. (I'd essentially have to unpack the pixels at that point anyway, or do some really clever stuff, comparing multiple pixels, which sounds fun but exhausting)

If I wanted to do this, would I be looking at a fork of the whole skia-project, and possibly coding some stuff in Rust, or is there some other way to plug in a new ColorType implementation?