I've no issue with a difference in colour in artwork that is old but where separate pieces have been stored differently.
I am quite busy lately, and occupied with other things ...
I suppose you know what a tryptich is - akin to a window with shutters, with the wing paintings attached to sides of the central painting. For most of the time, being the central piece of a church altar, they are either exposed to sunlight together, or covered together. It is the UV content in the sunlight that causes the darkening. Thus I see the obvious difference in brightness as proof they were not created together.
The central piece could in theory be faithful copy of the original - but I think it is not.
Not only is there no smooth transition to either of the wings. Persons are "cut off", the cave ceiling doesn't really match, and body proportions seem somehow off, and the clothing style looks different.
PS Stepping into the realm of speculation, I would guess that the creation of this triptych is a money making scheme.
I would be a bit more generous ...
Such triptychs generally adorned churches, which were central part of the daily life back then. The church itself, i.e. the building and its decoration, were a perfect opportunity to show off, and demonstrate you are more successfuly and better off than the other town over there. For them it was not a great piece of art and cultural importance, but of craftmanship that had to "keep up with the Joneses".
Thus I believe they had no problems repairing a recreating parts of it, if it got damaged. Society hasn't changed that much, I think it's established fact that the people depicted were not imagined ones', but real persons served as stand-ins for this religious motiv. Most probably a "noble" as Jesus, and the city council and most successful merchants surrounding him. After all, they donated the money, and thus they decided whom to depict. And I guess one could find the artist himself somewhere in the background ...
Painings itself are not really an area of expertise for me. I'm sure the position and configuration of the nails front right is supposed to tell us something, as are the usual gestures, finger pointing, clothes & colors, and background objects. But I leave that to the experts.