Any idea how I can get these shirt collars back to white?

[AintNobodyHereButUsChickens]

DH needs a suit next weekend and he’s just taken his shirt out of the wardrobe to make sure it still fits and discovered the collars on his shirts have gone yellow! This is the worst one. How can we clean it back to white?

Rub in washing up liquid then put in washer, i use a scoop of oxy clean or napisan with my whites. Have just successfully got yellow suncream stain out of dhs white teeshirt collar.

I find that adding oxy clean to the wash in the machine isn’t quite as effective. Put it in a bucket with oxy clean and soak it first. Then wash.
I’ve just got fake tan stains out of pretty much everything DD touched by doing this whereas putting straight in the machine left some marks.