If you get a chance sometime with a dead carto, take the polyfill out, wash it, then take a flame to it. You'll notice that it doesn't char, it melts.
But, if you hold it about a foot above a match that you just lit, you'll notice it turns darker. The reason for that is because of the impurities from the match is staining it, but not burning it. The "char" will just be soot deposits, not burn marks.
Juice doesn't completely vaporize away. There are some things left behind in the process, and this is what darkens your polyfill. If you use clear CE2 or tanks, you'll notice that the juice in there also becomes darker the more you use it. This is because of the same reason, leftovers.