You can airbrush Citadel paints, I've done it before, they are not the easiest paints to work with when it comes to airbrushing in my opinion. I've had success thinning them down with a combination of alcohol and water. They can also be a royal pain to clean out of your airbrush.

If you're starting out airbrushing I would recommend Vallejo model air in combination with the Vallejo thinner and airbrush cleaner. Also Tamiya acrylics, while not having the most diverse color range, they are the nicest paints to work with when it comes to airbrushing.