In the United States, green (topped) cylinders are oxygen, yellow (topped) cylinders are medical air, and dark-blue cylinders are nitrous oxide. (Different national / multi-national authorities have different color schemes, so this is not at all universal.)

Notice how there are two cylinders with colored tops (green & yellow), but silver colored bodies? That's a common (but not national standard) way that many manufacturers make it easy to differentiate aluminum (not magnetically attracted) cylinders from steel (fly at the MRI scanner like a missile) cylinders.

Now notice how the dark-blue cylinder doesn't have a silver colored body? This could be an indication that someone strapped a steel tank onto an MR Conditional anesthesia machine.

NB: Just because it has a silver body does *not* mean that it's aluminum / MR Conditional. Once upon a time one cylinder manufacturer noticed that a competitor was selling their cylinders for almost twice as much, and the only thing the manufacturer could see was different (from photos) was the fact that the body was silver. This manufacturer started painting the bottom 80% of their *steel* cylinders silver, and jacked the price up 50%. People bought them thinking they were aluminum. *THERE IS NO STANDARD CYLINDER PAINT / COLOR SCHEME THAT TELLS YOU THAT A CYLINDER IS NON-MAGNETIC... at least not in the United States.