What is a neutral colour?
Is it a pH=7 colour? A transparent colour? Greenish... brown? An oil colour... or maybe acrylic?
Conservators mean by it: "I will paint the missing area with a single colour which you won't see much". It can be green, purple, pencil coloured or either with acrylic paints... Controversial is on the cards and whatever we do, even when not doing anything at all, conservators know that our decision will not suit everyone's taste.