If you varnish
Douglas Fir.Pine,Oak,Maple,etc. it will darken over time-both indoor & outdoor.
My 1953 house is full of D Fir & pine along with maple floors.
All have gone very dark-especially the D-fir trim.
This leads me to believe that there is more to this darkening than just the wood or just the varnish
If I scrape the varnish off with a cabinet scraper,the varnish flakes are quite yellow. The scraped wood becomes several shades lighter-not white like new-but it does lighten substantially.
I have to suspect it is more varnish darkening than actual wood darkening.
Just a note: Some old carpenters in my area took to adding a touch of white paint
to their varnish to combat this darkening & it helps quite a bit.
The resultant color is not a "natural" wood color,as we know it,but it stays light & is a help in interiors that would normally turn very dark (& somewhat gloomy)