why do white shirts turn yellow around the collar?

My white shirt turned a light yellow shade (sort of like a pastel yellow hue). And it was only the collar that turned yellow. I didn't do anything out of the ordinary compared to what I usually do, except that I used warmer water.

I tried bleaching it, but it didn't make any difference. I do have some Borox that I could try using, but I'm not sure about the correct mixture. I've also heard that oxiclean is also good for this purpose.

Like I mentioned earlier, I've never had any problems when washing with cold water, so maybe it was the use of warm water that caused it? Any assistance would be highly appreciated!

#AskLemon8 #clothes