Ah, okay, so it's no longer possible to go with that one skill panel that automatically adapted its colour to whatever the "theme" colour of the main tileset was? Like, for example, Builder staircases on the Dirt tileset are gold-yellow, so the skill panel would be as well; then, the next level is a Fire level (blue staircases), so the panel turned blue, and so on.
I've been enjoying this as a cosmetic feature quite a lot, but I guess it relies on the now deprecated "mask" files?
If this must be decided upon on a pack level now, even though I'd consider it a step back from what we had before, I'd probably go with a slick, clean black for the upcoming Lemmings Open Air. Although a case can be made for just sticking to the original one, since that pack in particular is all about "being as authentic (visually) to original Lemmings as possible, while providing puzzles of NeoLemmix quality".
For my existing / converted packs in New Formats (Lemmings World Tour and, soon, Pit Lems), I haven't made up my mind yet on which colour would work for a unified skill panel.