I've got several light markers that have become red in my file and even the new lights I place are red. Some of the lights that have been there since my last save are normal looking and cast a glow like normal, but the markers that have become red don't shed light and any new ones are similar. Any ideas?

I know you can't have too many shadow casting lights in one cell, I don't remember the exact limit. Some of mine turned red but i deleted all of them and chose new lights without looking to see if they were shadow lights.

EDIT:
Just did a little experiment and put a bunch of shadow lights in a cell. After placing 4 lights I started to get red ones. If you're not using shadow casting lights then it's something else I don't know about.

Hope this helps.

Do you have a custom shader? Light markers that cast shadows are red for me as well and I use a custom shader. I think that's the problem.

The 4 shadow casting rule thing confuses me. Is it really per cell? If we have rooms defined so that no more than 4 render at a time, could we have more than 4 then within, say, a single dungeon?

If its 4 per cell that seems extremely limited and I can't believe the entire of Skyrim was made under such a limit.

I don't think it's per cell. As far as I understand it, it's per user's screen. If some camera angle has 10 shadow-emitting lights visible, not all of them will cast shadows, for example.

As Adolon says, it's 'Per users screen'. The game renders everything in front of you unless it's culled (Not rendered) by a roombound. This means that if you had a cell that winds a lot and has lots of turns and roombounds, you could put many many shadow casting lights in it. It's just 4 can be rendered at once before it goes bonkers.

Most of my interiors have like 7-13 shadow casters, depending on the cells size. I use shadow casting lights for about 40% of my total lights.

Only use them where it really helps. If you have an open smooth hallway with nothing to cast a shadow, don't use a shadow light.

Thanks for the info. How does the CK handle this though? From what I've seen adding more than 4 makes them red as the OP describes, so placing them would be difficult if we can't see what light they will cast? I've not really played with lighting properly yet so maybe it doesn't work as I've read here.

Once you are done with a room, hide the light nodes you don't need to see by selecting them and pressing "1". This will hide them till the cell is reloaded, allowing you to continue lighting your cell.