Ron, I don't own a Workman's and am not really familliar with UK so I have to ask. Do they treat the material? Some such treatments will repel dye the same way it repels stains.

Can you get a swatch from UK? Seems to me that would be the way to go. Test it first.

Mychael mentioned a couple of points, getting the color intensity you desire and using a mordant to set the color. His point of using boiling water shouldn't be an issue using preshrunk cotton.

To extract sage you will need experiment but if it is like walnut husks you'll need to boil about as much sage as you use water, if not more. Depends on how dark you want it to come out.

Being a little older, I remember when dyes were not colorfast, neither was a lot of the clothing being sold at the time. I remember my Mom using salt to set the color. She would add a cup of salt to cold water on the first wash, never seemed to be any problem after that.

Don't know if any of this will be of any help to you, just a few things I've picked up along the way. Give it a try and good luck, I will be interested to hear what comes of it.

Mike