I seem to recall that nitrogen gas has been delivered to me in both green and/or black tanks. Green seems to be the primary color I've seen though. We use the gas in broadcast antenna installations to purge and pressurize the feed lines going up to the antenna. Off topic, sorry.

A welding shop should be able to help you. Generally they trade out the empty tanks for full ones, and you pay for the gas. If you want different gas, I'm sure they'll sell that to you, and you may or may not need to pay the difference in the cost of the tank.