I use Quick Silver horse shampoo, does an excellent job on white birds and doesn't dry and fray the feathers like blueing will do.
The key to white birds is keeping them clean prior to washing, once stained for any period of time the stain is all but impossible to remove completely.
When using whitening shampoo be very careful not to stain your birds blue or purple, the key is to wash your bird first with a good quality shampoo, I use Corona Horse shampoo with lanolin so as not to dry out the feathers, after rinsing the first shampoo out I place about a nickel to quarter size drop in the palm of my hand of the whitening shampoo and lather it up in my hands first then I apply the lather to the exterior feathers only. I rub it over the entire bird from head to tail and let it sit for about a minute or two at the most then thoroughly rinse, after rinsing I place the bird in a cage in the sunshine to dry out and this seems to make them brilliantly white.
Of course others have their own methods but this I have found works best for me.
Blessings,
Bo