Many panel covers ARE painted in businesses.
With that said, I would guess that would leave the panel manufacturers the choice of not painting, painting a specific color, and/or painting with something which will protect the metal from rusting and so forth.
I would think they would want to paint it with something to protect from rusting.
As for color, I would imagine they would want to use a "primer" color. Then people could easily paint it. Why primers are the colors they are, I have no idea?
Anyway I found the following which might be some reasons for this...