Plants Fading / Yellowing Over Time

Many of the plants in my terrariums fade / get yellow over time, but I can’t figure out why. Any thoughts?

Pic 1 is of pilea baby tears a few months ago, nice and green. But in pic 2 you can see all the leaves inside the bottle fading, whereas the ones above / outside of the bottle are green.

Pic 3 is of a red-veined fittonia that’s faded. Pic 4 is of a parlor palm. Sorry if those are hard-to-see.