I use good old-fashioned bluing for that. But if you let your whites dry in the sun, they'll stay a lot whiter than if you use a tumble dryer!
I had to look up what bluing was!
I think she was talking about soaking them in a borax/hot water mixture to take the grey out and return the whiteness. Although I'm not sure if you can just buy borax in a shop here or if it's something that you have to order online from somewhere?
I don't have much problem with whites turning grey so it's not a solution I've sought out, but just thought to mention it here. I've heard that can happen (whites turning grey) where the water is harder, and the detergent isn't washing out thoroughly - but we have soft water up here & I'm glad.
I've heard if you have hard water, you can add borax and also vinegar (like apple cider vinegar) to a wash to counteract the effects of the hard water.