I have a pair of black RTW jeans which had been getting greyer and greyer, but the fit was still reasonable, compared to other purchased jeans so I decided to overdye them black.
I've used the Dylon all in one machine dye in black - this already has the salt mixed in, unlike the normal Dylon machine dyes where you add the salt yourself.
One pack of sye colours one pair of jeans, so they are in on their own and I will check on them shortly to see how well it has worked.
I have used the other type of Dylon machine dyes before and as long as you follow the instructions and don't try to dye too much at once it works very well on cotton fabrics. I had patchy results in the past with the hand dye, but I think that is becuase the liquid stays still.
This dye is designed to be used with a front loading washing machine (the normal sort in the UK). And how you use it as follows:-
Put the powder into the machine and make sure it falls through the holes in the drum.
Add the jeans/fabric and put the machine on to a 40 degrees Celsius cotton wash with no detergent.
This is the dye cycle and looks pretty scary as the water is black!
When this finishes, add a little washing poweder or liqd and run another 40 degree Celsius cotton cycle. This makes sure all excess dye is removed from the garment/fabric. You can then take it out and dry it. Probably best to wash with other dark things the next couple of times.
Last run the machine again on another 40 degree Celsius cotton cyyle to make sure the machine is completely clean.
After this you can use it again as normal.
My machine has a double rubber ring sealing the door and some black liquid was in this but I just wiped it up with a cloth before the final cycle.
Dylon have a guarantee on the package saying that it does not harm your washing machine.