But trevanian, this is not the first time we see the corridor outside of Engineering. The first time is earlier in the episode, when Kirk and Scotty are checking out the situation of the locked door. Following is a cap from that scene and you'll see the hallway is definitely grey. If you go to Trekcore and look at other caps from that scene, some of them have a blueish tint to them, which could definitely be the gels you mention, but the corridor isn't purple.
You can argue that Finneman used a purple gel for effect in this one scene and I can't argue with that. None of us can. He's dead and there are no records to state either way. Or you can argue, as you do, that they meant for the corridor to be purple and it was only used for this one shot. That's a tougher argument to make as other shots from the same episode don't back up this theory, but again, no one can argue against it. A much simpler argument is that the corridor was always meant to be grey. It shows that way in other shots in this same episode. It is grey in other episodes. But far be it from me to keep a Trekkie from getting out on a limb to make a point. That never happens here.
You can argue that Finneman used a purple gel for effect in this one scene and I can't argue with that. None of us can. He's dead and there are no records to state either way. Or you can argue, as you do, that they meant for the corridor to be purple and it was only used for this one shot. That's a tougher argument to make as other shots from the same episode don't back up this theory, but again, no one can argue against it. A much simpler argument is that the corridor was always meant to be grey. It shows that way in other shots in this same episode. It is grey in other episodes. But far be it from me to keep a Trekkie from getting out on a limb to make a point. That never happens here.

