LNWR Wagons Vol One says:
"From 1850 onwards the bodies of goods wagons and vans were painted a medium lead grey colour. It has sometime been termed 'invisible grey'. a 'lead colour' or even 'light grey' in some references. The grey was mixed from equal parts of black and white pigments, and was slightly darker than LMS grey."
While this is OK as far as it goes, the colour would be different if the pigments were measured by weight or volume. What is certain, though, is that the white lead in the mixture would oxidise over time and the colour get progressively darker between repaints.