Elwood Ive used the Ritt method a couple times.Last one was a4* 120 canopy.
This is a large canopy, so what I did was use an old army alum. kettle to boil the water, well not quite boil, that will melt the plastic . I didnt want to stain the inside of this pot, so, I got an old Igloo cooler I didnt worry about mine, but, you can use a large garbage bag inside to keep it from staining. I used black and blue also, anyhow I just poured the hot water into the cooler wwith the dye then set the canopy inside, closed the top to hold heat and checked it 3 hrs later, ended up leaving it all night and 1/2 the next day and its still not real dark.
I also tinted a Something extra canopy a couple years ago and it didnt turn out nearly as dark as Id like.
Some types of plastic take the dye better than others Im told, however, I personaly am done with Ritt.
Im thinking of using a spray product next time.