the colour loss is only obvious from an acute angle
This seems to suggest that the colour difference could be due to a change in the surface texture of the wood1.
While this may be part of it as you noticed some faint brownish colouring in the water obviously something coloured was removed from the wood.
From googling around, it sounds like the colour I've ...
You've simply washed off some of the top layers of oxidized wood, exposing the lighter wood underneath. It'll oxidize over time.
Will it eventually match the oxidization around it? Probably. But since this is a natural product undergoing natural processes over time, who knows?