If you want to get technical - using the fixed colors in terrain works visually, but not physics-wise (the pixels using the fixed colors will be treated as non solid).

Night of the Lemming Dead (Zombie level, obv)

This was the title of one of the first two zombie levels ever released (from Lemmings Plus Omega; the other one was culled making NotLD the oldest surviving one).

The intent behind Paint was to eventually cull only on terrain as a setting, with objects intended to be used this way made into paint objects. Given that never went ahead, and SLX presumably wants to go the other direction on how only on terrain is used, it's probably not worth keeping in SLX. It may be worth flagging such objects in some way so that when NL levels using them are imported, the only on terrain attribute is applied.

I wonder if Save Me, and the 100% solution to Cascade, are also worthy of mention?

Rather than the quick change, what if the color change slowly moves down (or up) the lemming's body?

I've created the Bugs & Suggestions board for you.

Using a proper bug tracker would be ideal in a perfect world, but we must consider the actual situation: People don't want to sign up for and use a seperate bugtracker. They want to report the issues here on the forum that they're already registered for and already familiar with. A seperate sub-board is not a huge hurdle; an entirely seperate website with its own accounts and UI is, especially for people who are not experienced with such sites (most of our users are not developers, so they would not be familiar with GitHub or BitBucket's issue trackers).

Then, as a developer, at least personally I found - it was much more hassle trying to duplicate the reports and all important info over to the bugtracker, and seperately keep tabs on both sides. Having just one place to do this is far more convenient, even if that one place is the less convenient of the two possibilites for me personally. Thus why I eventually went with the board here as the sole place for this. A further benefit is that it does not require the user to know whether any given issue is an engine or editor or stock content problem, in the cases where the line is blurry.

Not sure about the need for a seperate releases & update board. Even NL and Lix don't do this; we just use sticky (+closed, in NL's case) topics on the main board.

If you're getting to the point where bug reports are cluttering up the main board, then certianly, a dedicated Bugs & Suggestions board is a good idea. Likewise for Level Packs. If you feel one or both of these are getting to the point where topics relating to them are cluttering the main board too much, let's go ahead and create one/both (as applicable) of those subboards; whereas if it's just for the sake of having them, let's not for now.