So, why would they do that?
I’d expect it in a magazine, but on the walls?
Why would restorers outline the figures on the walls in white?
I do not really know, but in this case I would guess it makes them easier to see clearly.
I don’t know this for a fact, but I would guess that those outlines are just a temporary part of the restoration process. Something like putting masking tape down to shield one part from drips and nicks and dust. When the whole thing is restored -— and it’s a painstaking process -— I’ll bet the white outlines are removed.