Most woods will darken somewhat over time depending on level of exposure to the atmosphere and UV radiation, though a lot depends on species and finish applied. Some tend to go the other way and will fade if overexposed to direct sunlight.
It seems that vintage examples of this great design were available in teak and possibly birch(?). Good close-up photos of the wood parts of your chair might help to identify it and help predict to which shade it might eventually turn.
Nice looking chair and flat, BTW. Is that an automobile dealer across the way?
as tktoo writes it came in different wood types google image and you see different woods and collours
it seems a bit light for the wood wich in your case does not seem to be birch I had a birch one and that was much lighter an yellower
take a close up picture and you will be helped here i am sure
if youu want to know wich wood types were used so 'what is"original"simply send a mail to gelderland the producer then and the company still exist There is a very gentle person there who will surely help you He responded to my emails wich questions on originality and types/modelnames of gelderland (vintage) chairs. His name is Evert van Dijk
ps I take it you know the chair was produced by Gelderland
BTW looking at your view it seems you live in Utrecht (or Haarlem) looking at the facade
maarten
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