Both short and long forms can be used when an adjective or a participle is the predicate:
«Он убит» = «Он убитый», «Он зелен» = «Он зелёный».
But adjectives tend to the long form, while passive participles tend to the short one.
«Он убит» is used much more often than «он убитый», but «он зелен» is used much more rarely than «он зелёный».

When an adjective or a participle is not a predicate, the short form is never used.