Mate, I've dabbled around with dying feathers over the years.

In all honesty, it is a royal PITA.

I used to tie quite a few flies for a shop I worked at for pocket money and needed specific colours for traditional old fly patterns.
I'm going back more than a few years trust me

Started out with white feathers and used a commercial dye available at pharmacies in those days in 60 degree water to start the process.
Had to leave the feathers in the solution for 15 minutes and then remove them to cold water for the dye to set. The hardest thing with all this was getting the feathers to dry again.
It was fine dying full or half capes, but I did lose some because mould set in if they weren't completely dried.

I was shown by a taxidermist to put salt on the capes after they were dyed.
It was also supposed to set the dye and absorb any remaining water - this worked well.

It was a long process and messy to boot, but it did work.

I'd be very surprised though why you would want to dye your own these days as there is every conceivable colour under the rainbow from online stores, unless you have something special in mind.

That's just my take on it though.

Good luck mate........oh, and work outside when your dying your colours.
That stuff stains like a *******!

I had a go at it last year with what they call in the US the "Kool aid dying" method.