Coming back to this topic for a moment (since it's a possible feature of future NL), I've read the OP again and it's worth keeping in mind that this is stemming from {content creators not providing zombie/neutral recolourings for their sprites}.
Forcing a default spriteset seems way too heavy-handed a way of dealing with this, when it's something that could easily be dealt with by simply communicating with the content creator and asking them to, hey, include recolourings for their sprites!
When I created the Lemminas sprites, I made sure to include recolourings for zombie, athlete, and selected lemminas. It seems unfair to punish someone who has taken the time to make sure they've done it right for the sake of resolving an issue caused by less careful content creators.
Perhaps a better way of dealing with this would be to build some sort of "spriteset recolouring tool", which allows users to create their own custom recolourings, whilst also making
sure that they also apply recolourings to the various states. For example, it could be that it spits the resulting PNGs out with certain code allowing them to be used by NeoLemmix; if someone doesn't run their sprites through the tool, then they won't work in-game.
If this seems equally drastic/too much work, another idea would be to have the option to force a default
recolouring, rather than an outright default sprite set. This could simply apply a set of colours (green/brown for zombies, white/grey for neutrals, for example) no matter what the initial colour of the custom sprite is.
Meanwhile, regarding another related issue (
completely blank lemming sprites which don't even show up in CPM), I wondered whether the lemming's position could be indicated in CPM (via the pink dot) which would be a way to get around that particular problem, should it ever arise. And, by extension, could the dot itself be recoloured to indicate zombies, neutrals and athletes? This would also help towards resolving the OP's issue.