From 'Some small birds live on honey' and 'All birds that live on honey are colourful', we know that those small birds that live on honey are also colourful (since all honey-living birds are colourful). Therefore, some colourful birds are small birds (specifically, the small birds that live on honey). Option B is incorrect because we only know that honey-living birds are colourful, not that ALL colourful birds are small.