They dip them in the acidic solution, then whilst out of the water they inject colours under thier skin with a syringe. After a few injections which may take 45 seconds, they put them in the solution and or tank water again, and repeat the process untill it had finished. By that time 3 in 5 fish die from this within the next 2 weeks.
The dye wears off in 3-4 months. I've also heard of 'Dyeing Stations'. Were people bring in thier fish to be dyed, come pick it up later and pay the man some cash.