Idk if this is considered a base/mutation since it could be both, but I thought I would suggest it! I also completely understand if no one likes it lol

Phasing is commonly seen in wolfdogs (if they have arctic wolf in them) and arctic wolves. They are born with dark fur, but the color lightens the older they become until they eventually turn white or a light grey. I'm no expert on this at all so I'm sure there are more than enough folks out there who know a heck of a lot more than me haha.

As far as the lioden lions go, maybe the cubs with this base/mutation are born black but lighten to either a light grey or white when they reach a specific age. Perhaps they don't have to be just black, they could also be blue, brown, golden, etc but eventually turn white.

I also understand this might require a lot of coding, so again it makes sense if it get's rejected. However, if it's just kept as a base it could look really pretty and natural (if someone wants to draw it if this gets enough positive attention that'd be cool)

An example b/c I have no idea how to draw a base lol (or you can just look up a picture of it):

http://texaswolfdogproject.org/resources/phenotyping/what-is-phenotyping/phasing