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?

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