Greetings
We have 2 separate problems on leather couches.
The leather is fairly thin medium-to-dark brown.
In one place, there is a spot where it has gone yellow. And it feels sticky. My wife, the pretty-much-everything maven, cannot get this cleaned up. It looks like a bleached patch to me. Can't get the stickiness off it, and dyes don't take on it. (Roughly 3 in. X 1 in.)
Unclear whether it is something still adhering or something bleached out or some inner chemical property of the leather.

Problem two is on another couch, same leather, where there is what so far is an indelible dark stain.

If pix or details I can't think of will help, we'll provide them as possible.

Thanks for being here to ask!

denno