The thread that's linked in above says he used the hottest tap water his faucet would give. Most hot water heaters are set in the 130-150 degree range.

The Rit dye packages say to use hot water and mention adding salt and a touch of soap.
I used two packages of black, one package of Navy blue, the right amount of salt for the 3 gallons of water I used, and a touch of soap. I had to put in a new hot water heater a couple of months back and it's supplying water that "steams" and you'd not be able to keep your hand under as it comes from the tap without burning your hand. I was a bit worried that it'd warp the two canopies I tried to dye so cut some off the flanges and tested it under the faucet. The Rit instructions mention stirring the stuff so I did about every 5-10 minutes. When the two canopies came out of that dye bath after over 3 hours, they rinsed off as clear and undyed as they went in. I think the 3 packages of Rit dye cost me about $10. That was $10 down the drain. Wish I'd thrown in some shirts or something so the time and money weren't so completely wasted. I also bought a new bucket for this. At least I now have a new bucket.

It's a dead simple process that works when it works, and is a total waste when it doesn't. And nobody has said boo about how to predict if your canopy is one that'll dye or not.