Previously it was found that the similar D-316 concentrate went orange after 1 year exposure to air and Mark suggested the color could be used to estimate any needed increase in developing time.
I made the following 3 separate solutions in deionized water and exposed each to air oxidation for 37 days:
Sodium ascorbate 12g/L
Sodium ascorbate 12g/L + Sodium sulfite 20g/L
Attachments show that the solution containing sodium ascorbate alone went orange, strongly suggesting the orange color is due to the formation of an ascorbate oxidation product, it does not come from phenidone.
Addition of sodium sulfite to sodium ascorbate apparently prevented the formation of the orange compound, perhaps that is why it does not show up in oxidized Xtol.
There is some recent research on ascorbate oxidation in the human retina and also in non-enzymatic browning in foods but I did not find any mention of an orange compound so cannot say what it's name may be.