What gives color to most of these plants are carotenoids (or carotenes according to Wikipedia). I've studied them as some form of lipidic substance, which doing a quick research doesn't seem the most common denomination though.
Basically carotenoids are molecules that absorb light at spectrums of light ranging from ~425 to ~500 nanometers, which is whithin our visible spectrum.
Carotenoids have two very special properties, the first as pigment, the second as antioxidants. However in this late group they can prevent oxidation only partially. From this point on it gets really funky, because it meddles with the fact that you have different types of O2 which I don't know the names in English, but basically you start with 1 O2 which is very reactive (and therefore oxidation happens really fast) and carotenoids can help through photooxidation or perhaps autooxidation for them to become 3 O2, which essentially slows down the oxidation process.
There is another process through which this happens, but basically what you have to know is that when carotenoids break down through this process they cease being antioxidants and activity resumes in your food. The more carotenoids break, the browner/darker the bell pepper gets because the pigmentation disappears in favor of other coloring substances.
Then there's the Maillard reaction which is what happens when for example meat ages. A LOT of people think that meat oxidates but it's not true, what happens is that myoglobin changes of shape (to simplify it) into metamyoglobin. It's also one hell of a chemical reaction and these two are arguably the two most complex reactions that take place in food.
I did this from the top of my head (only studied this stuff on a basic level) , I might have gotten a lot of things wrong so please if anyone sees any red flag pitch in to correct me. As always I encourage ya'll to make research on your own.