Yellow leather Toadstool turning brown

[Goomba]

I think it's pretty obvious the store you bought it from was misinformed or lied to you.

You saw it totally white in a store, brought it home, it got darker after some time, then after more time got even darker. That is a perfectly typical experience with a bleached coral, white in store>>yellow later at home>>brown later on.

The picture posted is what you can expect from a fiji yellow toadstool, if you see them brighter it is because they are stressed out and either in the process of bleaching or recovering from bleaching. You cant force them to stay that color, they will always drift one direction or the other depending on if they are happy or not. It is also possible you've seen dyed ones, the local coral collectors used to dye them yellow so people would buy all the brown ones when they ran out of nice ones to find.

Your expectations of a leather coral are too high. There is no such thing as a permanently fluorescent yellow toadstool.