The LEGO Brand Rules state:
No modification. Do not change or otherwise modify the shape of any existing LEGO brick, element or other LEGO content, for example, by stretching, cropping or recoloring elements.
It states you cannot recolor elements, although I noticed that in Suburban Prop Gallery asset inventory there’s a 2x2 curved slope that colored Reddish Brown, but that piece has never been made in any brown color.
So are you allowed to color pieces so long as It’s a color used for other LEGO pieces?