(Copying this from a post I just made on another board)
I recently dyed some diapers in the washing machine, and they came out beautifully even as well as a gorgeous colour. This will work particularly well if you want all the diapers to be one colour.
To do this, I figure that a washing machine holds about 20 gallons of water. Fill it with warm water, and add about 20 cups of non-iodized salt. (The salt helps break the water tension so the dye will soak throughout the diapers. You can also add calsolene oil for the same purpose; it only takes two tablespoons per washer load.) Let the washer agitate enough to dissolve the salt thoroughly, then stop the washer. Add dye (you'll need a total of about 8 tablespoons of dye for a full washer-load, if you want a medium shade; dissolve it in water before adding it to the washing machine, and test the color on a scrap of fabric or paper towel to be sure you'll get the color you want. Remember that colours will look darker when wet than dry, so dry your fabric scrap/paper towel during the color check to be sure you have the color you want). Turn the water on and let agitate briefly to make sure the dye is dispersed throughout the machine. Add the diapers.
You'll need to let the diapers agitate in the washer for about 20 minutes; make sure you keep stopping the washer and returning it to the beginning of the agitate cycle before it starts to drain, or you'll lose the dye down the drain. After 20 minutes, add 2.5 cups of soda ash (dissolved 2 tablespoons to one quart of warm water). Add the soda ash slowly -- a little at a time over the course of 15 minutes -- and use a stick of some kind to hold the diapers to one side as you pour it in.
After you've added the soda ash, let the diapers agitate in the dye bath for another 30-60 minutes (less for lighter colours, more for darker ones). Allow the machine to run its regular rinse cycle then. Wash them diapers on hot with four capfuls of Synthropol. (You may need to do one extra wash with Synthropol to make sure the diapers will not bleed further.)
This sounds like more work than it is. I highly recommend this method, as it's the least work of any of the methods I've tried, and produced the best results.
Things to remember: wash the diapers in advance. If they're new, wash them until they'd be ready to use; if the diapers won't absorb water yet, they won't take the dye, either. If they're used, do a prewash of clean diapers with Synthropol, to make sure there's nothing in them (including detergent residue) that will keep the dye from taking. You may need to do a strip wash if you have detergent buildup.
diapers to dye
noniodized salt (about 8 normal-sized boxes will equal 20 cups)
powdered fiber-reactive dyes
calsolene oil (optional)
Synthropol textile detergent
All but the first three supplies are available at www.dharmatrading.com
. These instructions are based on the instructions for vat dyeing from their site.