A good question was brought up in Rebelscum's latest Question Mark. Are you putting your toys at risk by storing them in plastic bags? I know the easiest way to keep my Attacktix organized is with a few dozen Ziplocks. But this may not be the best way...
[It was stated that] putting figures in regular Ziplock bags could damage/discolor them rather quickly. Does this have to be under certain conditions only? Will this damage only occur if they are in the bags out in the light?
As plastic deteriorates it puts off gases. In an air tight container (like a Ziplock bag) the concentration of gases can speed up the deterioration of the plastic. In fact, the deterioration of the relatively thin (unstable) baggies themselves can contribute to the problem as well. Even in a cool, dry environment, figures sealed in an airtight bag will deteriorate faster than if they were left out of such bags. Every situation is going to be different, but in all cases you're better off not placing your action figures in airtight bags. Sturdier (more stable) plastic shells, or acrylic coffins are a much better storage option, but even these should be watched carefully and changed once every couple years or so.
We all have figures in our collections that have a slimy, shinny, or mottled look to them. This deterioration is a direct cause of the chemicals within the plastic they are made of and in the end inevitable. There are other elements like mold that attack from the outside, most commonly attributed to the transfer of oils in our fingers by touch. Given time and given human contact these are all things that will effect plastic toys and detract from their appearance. The best environment to slow this process down is a cool, dry, and dust free environment; something that is at times difficult if not impossible to provide. While Ziplock bags may seem like a viable option, they can hasten the decay we are trying to prevent and should be avoided whenever possible.
That being said, if you have your collection in baggies, and there doesn't appear to be any damage, don't panic. Keep an eye on things, understand what 'could' happen, and do what works best for you. As I said, the deterioration is inevitable; it will affect different figures differently, but with proper care you can at least take steps to slow the process down.