Hi Carl,
I've been meaning to see what I can answer, here. I suppose the easiest and least costly method I've come up with is using simple clothes dye like RIT. The gray I start with is RIT Pearl Gray #39. From this base, you can add other dye colors. For example, if I want it a gray leaning toward a more brownish tint, I add RIT dark brown #25. More blue--add a touch of Navy. How much you add depends upon how brown or blue you want it.
At any rate--You don't need to use the entire envelope of dye but you an mix smaller batches depending how much you will dye. If you are planning on dying an entire skin, you may need more for a larger container to hold the skin. If you are planning on dying a batch of feathers you've removed from the skin, you won't need as much.
I usually start out with a few of the larger feathers for testing the color. I mix the color in a stainless steel pot to a fair strength. (What looks good to my eye) If mixing two or more colors, I use the base color and add the other tinting colors sparingly till I get a color in the pan that looks right. More water will lighten the coloration, and of course, less water will darken and intensify it.
Once the dye mixture comes to a boil to make sure all the dye is dissolved, you can turn off the heat. Wet and submerge one of the larger feathers, for a minute or two keeping track of the time.. Take it out, wash it, and look at the color. Dry the feather with a hair dryer and look at the color. If it looks dark enough and the right coloration you can proceed. If not, the first thing I do is adjust the color. If it needs more of a secondary tinting color, I add this. Retest using a new wet feather, again keeping track of the time. ( keep the dye mixture warm enough by reheating before testing again.) Once I'm getting the color I want, then I can adjust the lightness or darkness. If it's too dark after washing out the test feather, I add more water and retest. If it's too light, re-wet the initial feather and place it back in the dye for another minute. Take it out, wash it and check to see if it darkened or not. If it did, and it's what you want, fine, dye the rest for the total time the feather was submerged/ If it's not dark enough you may have to add more dye, testing feathers till you get what you want.
Once you've got the color and darkness and dye time determined, you can dye the entire skin or the bunch of feathers. Make sure you wet the skin and feathers well before placing in the dye bath. Make-up only what you will need to dye what you have. If you wish, you can get pretty technical by measuring the amount of dye powder in each color you use and the amount of water you are mixing in. For example for a brown gray I might use 1.5 teasp. of Pearl Gray and a quarter teaspoon of Dark Brown to a cup and a half of water for dying a small bunch of feathers. This way, I'll have a pretty close color match for next time I want to dye the same color.
When I do small batches of feathers, I put them into a wire strainer to wash and dry. The skin should be dried well by pressing in paper towel, then laid flat on newspaper to dry.
Years ago Eric Leiser used Clairol hair color to dye his dun necks. He also use to use a photo process using silver nitrate solution and developing chemicals to actually photo process the neck. With today's digital photography, it may be difficult to purchase photo processing chemicals, but Miss Clairol is still around!
I know this seems like a lot of trouble, but when you actually do it and get to know the process, it's not all that difficult. I've actually dyed feathers using vinegar and food coloring. No different than using Kool-Aid.
Mark