I went into castle volkihar to do the fight with Harkon, but when I got in, the walls and floors had been replaced with yellow. I'd see the odd bone and jug, and I'd see people, but it was very difficult to see where I was going, as the shadows are not visible. I can see lights, but that's all. I've checked it with Dragonreach, but that's fine. The Volkihar Cathedral where you fight the boss has the same problem. Any way I can fix this?

Probably a texture issue that could be patched, but you're on your own there.

Are you fast travelling to the castle location? Maybe try traveling to the boat instead and manually travel over. If not, try reloading a previous save.

If all those fail, the official BGS solution would be to start a new game.

The bridge outside is fine, just the interior. I'm trying to reinstall it now.

Edit: Fixed