I'm using some carts I bought off amazon (lol) and that's exactly what I thought might be the culprit at first as well, however, I read in a few different places that even the legit CCell carts and other high end ones do the same thing with homemade oil. I want to buy some nice carts and test it but I'm worried it will be a waste of money. I can provide links to the equipment I use if it helps.

Carts (These were taken off amazon so here's a screenshot. It is a little suspicious that the item was removed)

Syringe filters

Syringes

edit: I should add that I've tried multiple different batteries, some high end with higher voltage and some weaker ones, all to the same effect. The batteries also do fine with normal distillate from normal carts, it's only the homemade ones that have darkening issues. The one I have right now is basically pitch black, for example.

edit2: I know I'm a moron for getting amazon carts I was really broke at the time lol