Aye, Picric will have its followers while others palm it off saying it’s only a yellow dye.I honestly don't know is the straight answer, but people back in the day couldn't afford to discard stuff like we do now , so it's perfectly logical to think that maybe flies being kept for as long as possible would acquire a certain "patina" over time.
On the subject of Picric , I fish with a senior angler who uses the finest grey partridge feathers treated with it on a lot of his flies. He shortlines from a drifting boat and it works bigtime.
Who knows if it’s that critical but it’s not beyond the realms of possibility that it is a certain shade of yellow which the fish are particularly fond of at times.
The thing is though if it’s constantly changing colour with age then obviously ,it’s not always going to be that particular shade of yellow. I would say that limits it’s usefulness albeit the colour it turns into over time also looks nice...?