Yes, thanks for the pictures!!! They actually are quite helpful.

It sounds like maybe you need to look for another one of these nice white toadstools that you were lucky enough to find. Since under your care it'll turn into a true yellow toadstool. And then when it's back to its true yellow form, do some testing and logging of parameters. If it eventually darkens up again, do some more testing and logging and compare them from the earlier results.

I'm thinking that maybe a way to sustain the bright yellow would be a low nutrient high light environment?

Yes for the most part they are a "mustard yellow". However there can still be nicer ones. Think about some Grey Poupon dijon mustard, kinda plain brownish yellow. Then think of Heinz brand, a sharper mustard yellow. Just keep it in a clean, natural state and it will eventually show it's true colors whether they are nice or not.

The only way to save a browned out coral is if it's currently in a system with too high of nutrients and you then correct that problem, it can "brighten" up a bit. Chronic low nutrients may lighten it as well but that is going to set up other hurdles for you to jump. However if the system it's in has optimal parameters then that's just the color it is, sometimes you have to get lucky.

Yellow Toadstool Leather Coral
 

Finding another one is highly unlikely. I don't think the store realized they were yellow leathers since they looked almost bleach white.



I'm sure if the store realized it was a yellow leather they would not have sold me a large piece for only $30.00 tax included. If I knew then what I know now I would have bought the 4-5 pieces that were there.


Come to think of it I did see some semi white pieces just a few weeks ago. They were not the same white.