FP Inks that change colour when you don't want them to

I started keeping a fountain pen ink journal a few months ago, and one thing I started doing on the index page, which is in the back, is putting the name of the ink, a tiny swatch, and a second tiny swatch if the ink colour changes while it is in my pen.

I've noticed several inks that do not keep their colour if they are in a pen more than a week. The biggest colour changes I've seen are:

Robert Oster African Gold
Robert Oster Caffe Creme
Robert Oster Cherry Blossom
Akkerman Mauritshuis Magenta
Franklin Christoph Honeycomb

I have seen it mostly in the muddy yellow-browns that I love, but pinks are also colour changers. By far the worst two are African Gold and Caffe Creme, they became several shades darker and more drab. Is this a common phenomenon?

(Edited to add: it might be the pen! Mona⁷ suggested the pen could be allowing evaporation, which would darken the ink. That would definitely be more noticeable in the lighter inks too.)