Clear categories are colour, other aspects of appearance (μονοκόμματο, έναστρο), and geographical. There is also clearly a metaphorical use in έβδομο ουρανό and δημοκρατικό ουρανό. You may find έκτο ουρανό a bit puzzling (there is only one instance in the corpus). What do you think the meaning relates to? You might also want a category of time (νυχτερινό ουρανό) - can you think of other examples which are not in the corpus data that are possible in Greek?