Mosiac, my first instinct would be to use enamel paint, the ones that come in the tiny pots (non-aerosol) that you paint model railway with, but that might not be cost effective for a large number of tins. I've used old car paint in sample pots to paint metal before with good effect.
Found this link whilst looking that might be of interest to you. It suggests different paint types for tins and how to remove the old paint from the tin first to get a smoother finish...:
http://gomakesomething.com/ht/metal/prepping-tins/