So I did have a writing sample of Diamine Crimson from 1.5 yrs ago.
I wrote on the same paper with a pen that had had Crimson in it for about 8 months, let it dry. The ink in the pen for months was obviously more brown. Then I reloaded the same pen with fresh Crimson from the old bottle, again on the same paper, and it was normal red color.
So, the red ink turned brownish while in the fountain pen for several months.