
The VFT's shouldn't discolor unless the insects overhang the actual trap or they are dead. A VFT requires the insect to struggle to fully close and dissolve the insect. Of course if there are any chemicals on the insect that can change how things work. When a VFT Catches any insect they should put on a Burst of Growth and the number of leaves should increase in both Number and size. If that doesnt happen there is something wrong with the potting mix or there is fertilizer involved.

As for the Nepenthes and Sarracenia's they have a fluid in the bottom of the pitchers which is the same as what is inside a human stomach, it dissolves the fleshy parts of the insects and the leaf absorbs the nutrients while the Exoskeleton remains in the bottom of the pitcher.
Sarracenia Pitchers can laterally fill up with ants in a couple of days and be useless after that. Of course the Crows love them when they are like that they poke a hole in the bottom of the leaf and suck the remains out that way for their own food. Destroys the leaf and doesn't do the plant any good either as it's no longer getting the required nutrients to grow.
Currently Im taking some photos of a Nepenthes Pitcher and Flower Spike opening for a Book that someone is writing so I'll post a link to the photos which are one a day roughly every 24 hours showing the nearly fully developed pitcher opening to the fully open stage. Unfortantly the Beta Photobucket doesn't allow me to create any Albums at the moment so I'll have to post the individual photos.

Col