Colour changes may indicate some nutritional deficiencies...Amazon parrots need plenty of Vitamin A - so you could try increasing the food sources in the diet to include more of the items high in that vitamin such as carrot, sweet potato, kale, spinach, red pepper, mango and apricots, chilli also dandelion leaves- but ensure they are free from pesticides.
Has he had a moult recently...as that does cause quite a bit of stress he n their system, so perhaps a vitamin supplement would help.
I don't know if you have an Avian specialist vet local to you, or one who would give advice over the phone, but it might be a good idea if you can to arrange for a general health check up.
Are you able to post a picture...we love to see member's birds anyway, but a close up of the head to show the issue may help us with other advice.
A good shower was apparently in order!
Who knows what this stinker got into.
He had his shower but it still looked very bad. Later in the day it looks a little better. Still some grey but it does s now clear that it was not the feathers.
I was so scared. No am so embarrassed!