I moved into my current place a few years ago and my TF room has no light, and toys that I've purchased since moving in here have experienced no discolouration so far (touch wood). This is because I've completely blocked the one small window that the room has. At my last place, my TF room had an average size window, but even with the Venetian blinds closed some light would always bleed in. I tried to cover the gaps but I couldn't do it completely.

My current TF room has a tiny window with a solid roller blind (no slats for light to seep through). I've stacked some empty TF boxes in front of the blind as another barrier, then draped a wall-hanging (of Dinobots) as another barrier (with yet another Galaxy Force wall hanging to further block out any light leaking out from the side), then in front of that I've stacked a pile of boxed TFs as yet another barrier. And I can tell you that even in the middle of the day, when I switch off the lights and close the door, it is nearly pitch dark in that room. Also on hot summer days, that room stays cool. It could be over 45? outside but the hottest temp I've measured in there was maybe around 26?. That room has no air conditioning, but just by virtue of keeping out most incoming sunlight, it stays pretty dark and cool. It's dark enough to develop photographs in.


Left: my previous TF room which had a large window with a Venetian blind. I had drapped wall hangings on either side (but couldn't cover the middle), put pieces of paper and cardboard around the edges to block out some light, and also stacked boxes behind and in front of the blinds to block more light. But it wasn't perfect as light could still seep in and eventually, despite my efforts, some toys kept in that room did experience some photodegradation.
Right: my current TF room which has a small window covered by a solid roller blind. In this pic you can see the two wall hangings draped over and around the window. Behind the big drape are empty TF boxes stacked up. After this photo was taken I stacked up more TF boxes in front of the drape. I had eventually done the same thing at my previous room, but I the real difference is having a smaller window and the roller blind just works so much better than Venetian blinds in just block out sunlight.

Another option would be just to paint your windows black. The downside to having such a dark room is that photos and videos often don't show up well. And recently when I've been doing video conferencing my chat window always looks the darkest compared to other people's. I've put a small desk lamp in that room just to help illuminate me when I'm doing video chats (I shine it onto a white shirt in front of me so that the light reflects back into my face rather than blinding myself with the lamp pointed directly at me ). Had a video faculty meeting today - everyone else was in these nice brightly lit rooms whereas I looked like I was sitting in a basement* full of toys. I might take a screen shot next time I do a video chat to show you how it looks like.

(*despite being an upstairs room)