Unfortunately, I don't think you will find the answer to be entirely straight forward or simple. Sovereign Hobbies has an online drawing posted of KGV showing her in profile in 507A for hull and superstructure with a black waterline and Anti Fouling Grey for 1940/41. They also, state the steel decks are dark grey and the wood decks are dark stained.

Alan Raven (Ensign 1 King George the Fifth Class Battleships book) has both KGV and POW for the first half of 1941 painted overall in "Medium grey."

I have several black and white IWM photo's of KGV in the first half of 1941 including a large aerial photo of her underway taken on a sunny day.

For what an opinion of a black and white photo is worth, KGV appears to be painted overall in what Alan Raven described as "medium grey" and not the much darker 507A. All the 14 inch and 5.25 turret roof tops are painted dark grey and the wood decks are painted a dark stain and the steel decks appear to be dark grey. The after tripod mast legs below the starfish appears to be painted black to about the top height of the HACS and then medium grey to the boat deck. The After DCT roof top (only) is painted dark grey. ALL 14 inch gun barrels appear to be painted in a slightly less dark grey than the turret roof tops. The 45 foot motor launch I can see is painted in the same overall colour as the superstructure and shows a much lighter painted colour below the waterline. All the search lights appear to be painted either dark grey or black and all the shields to the UP rocket mounts are painted the same as the superstructure.