We've sort of tried to cover this in the past but I'm not sure if we really have a consensus.
The correct usage is absolutely "Adjective citizen". Common usage bears this out, with most combinations giving 20 times the number of hits for "Adjective citizen" rather than "Noun citizen". This is clearly shown if you compare cases where the adjective is less similar to the noun, for example British citizen over Britain citizen, French citizen over France citizen. Looking at how governments describe their citizens also backs this up.
I don't think anyone is going to object to commonly-used abbreviations like us-citizen and uk-citizen. The key should be understandability rather than obsessive consistency.