Paintings conservator Beth Courtney sent me this great picture showing the incredible transformation of one of the paintings in her care. I'll let her explain:
Our eyes and brains work together to make sense of things and often a slightly yellow varnish doesn’t make much difference to how we perceive the relationships between colours so we can still tell which areas are white, blue or green. But when a varnish becomes very discoloured our brains can’t remove enough yellow to compensate for the discolouration. Although we know that sky is probably a shade of blue, it becomes difficult to tell whether it ought to be bright or stormy.
As I carefully removed the varnish the true colours were revealed and the dark, orange painting gave way to a light, blue-grey toned scene. Even as a conservator and knowing how much difference removing an old varnish can make, the dramatic transformation of this painting was a bit of a surprise!"