"drag mr physical sky into a material slot and instance it there. then uncheck the inherit from mr sky. then drop the saturation to 0 and crank up the multiplier. i just tried it, gave me a white sky without affecting the color of the light in the scene.
as a side note, i just figured out that mr physical sky is a completely separate thing from the mr sun and mr sky. i can now get rid of all the blue shadows without affecting the color of the mr physical sky "background". i always thought my sky parameters and mr physical sky were the same adjustments.
my sky and mr sun are the actual light/color whereas mr physical sky is just a map of the background."