In a sense, you could still use either 'wintry' or 'wintery'. For example, you might describe a cold, miserable day in May as being 'A wintry day', but you would probably not say 'She was wearing a wintry coat.'
Of course, people would normally say 'She was wearing a winter coat', but that needn't prevent you from saying 'She was wearing a wintery coat.' There, you are adding the 'y' in the same way as we often do with words, as in 'It was a sort of pastelly shade' or 'It was an orangey-browny colour.'