There shouldn't be any reason you couldn't powder coat the case, which is generally the preferred/'premium' way of coloring a metal case. It's more likely to stick and last than paint, and you can often choose interesting multiple-stage powder coats that give different patterns or finishes.
The problem is that offering both powder coated and anodised finishes can become very expensive. Powder coats have a significant thickness, an anodisation layer doesn't, so close-fitting cutouts and reference edges might need different dimensions between the two versions. You end up with two slightly different SKUs, effectively reducing the cost advantage you get from higher order volumes.
Not saying it can't be done in general, but especially for a first run, only either of the two should be used.
@iFreilicht, I wasn't necessarily suggesting it as a manufacturing solution, but rather @neocmetal seemed to want a white case for themselves. If they were going to paint the case it would be worth looking into powder coating it instead, I think.