Getting feathers white

[CMOTdibbler]

So I washed Dhorse for the first time today, and as he hasn't been washed in a v long time, it was overall a very satisfying process, and he looks all black and shiny.

But, his feathers are still distressingly yellow inspite of using shampoo for greys on his white stockings, leaving it on, scrubbing, rinsing, reapplying etc.

Any top tips on ways to improve matters ?

Shall I try a different method on each foot in the manner of a Good Housekeeping test ?

It's not that Ben will go to any real shows, just that I seem to be getting horse proud, in a way that I never get about getting the house really clean