i've used the kool-aid method and it works well. you mix it with vinegar as stated above and it sets the color, kinda like easter egg dye. And yes it lasts. I have used it for trout and saltwater stuff. Preparing the material is almost as important as what dye you use. some feathers have oils on them that make them hard to dye. If you google : Dyeing natural fibers with kool-aid, you will find a site that gives a nice run down on the process, and it shows what colors you can make and how they look on wool yarn. Hard to beat the price when kool-aid is on sale for like 10packets/$1. No doubt the acid dyes give a deeper color though.