Yellow leather Toadstool turning brown

[Goomba]

A picture may be necessary to see the problem at hand. Actual yellow fiji leathers are fairly uncommon, not even sure if they can be collected anymore with certain areas going offline.

What that means to you is if it ISN'T a real yellow it could just be that it's gaining color back from bleaching out or if it's kinda patchy like army camo it was most likely colored and it's reverting back to normal.

If it IS a real yellow leather they tend to be prone to bacterial infections that manifest as brown edges and necrosis. You can snip off the infected areas with scissors or maybe dip it but I dont know what would treat it.