Hello hope this idea helps you. I had a similar problem but 90% of my yellowing was caused by using the wrong shampoo and conditioner. (sold to me by a hairdresser.) I have beautiful white hair thats been growing out for the past 14 months.
I tried the silver shampoos and found them to be very drying and the results not lasting. A friend of mine bought me some Tresseme moisturizing shampoo and conditioner with vitamin E. The very first use I noticed my white coming back. Now I'm on the hunt for some styling products. (I air dry my hair right now.)