I "paint" mine with a special mix of thinned Beechem layout fluid. (dychem doesn't work..too thick and can't be thinned easily)
I use a small artist brush with about 1/2 the hairs pulled out.
The dye likes to flow with the barbs a bit...depending on how wet your brush is.
It doesn't like to do the underside of the feather near as well as the top.
Stuff goes on easy.
Smell goes away completely after a few hours...doesn't smell all that much to begin with.
Seems to stay on the feather very well. I've been doing it for years..wet feathers..lots of shots..whatever..just stays on.
I've done different designs over the years...don't have many pics of em actually but here's a few..I've done some with colors other than black that have turned out kinda cool too..My fav was a grey feather with black and green...just no pics of it.