Its just genetics to be honest. Cornuta are brown to begin with then you add a brown cranwelli to the mix. It's still s nice looking frog
I find that the green ones keep more vibrant colors.
Agreed. Looks like it's just a natural change in colour due to age/genetics. Carotenoids might make it a little more colourful, but not enough to make it look like it did when you got it.