Wow! really really cool!!
Near my place there is a spot where the 'Yellow Stainer' (Agaricus xanthodermus) fruits. The yellow stainer becomes yellow when you cut it, and what makes it especially interesting is that the yellow pigment is an azobenzene derivative (4,4'-dihydroxy-azobenzene). Azobenzene is very well known in the field of photochemistry because it is a light-driven switch. But, in nature, it is extremely rare. I actually only know of this specific example....
So I am very curious about what the bright yellow is in this fungus....
According to this review on fungal pigments: Pigments of Higher Fungi: A Review
It appears that the main component of the yellow pigment could be a compound called 'gomphidic acid'
This molecule is very similar to the molecule from the lichen that you posted a few months ago, vulpinic acid, except that one of the phenyls has 1 hydroxide group and the other contains 3 hydroxides.
This is the structure of vulpinic acid, for comparison:
I still don't understand why Gomphidius has a yellow base, what about the metabolism of fungi makes them choose vulpinic acid-like molecules as pigments, and whether there is a functional reason why the yellow stainer uses an azobenzene derivative and this one a vulpinic acid... My current guess is that the metabolism to produce vulpinic acid evolved and is often recycled for pigment production, and that the yellow's stainer is using the azobenzene as some form of defense mechanism (it would be very cool if it turns out to be a phototoxic one) and the yellow color is merely accidental in that case.