I don't know if this is practical or not, but I think it'd be cool if the sky changed color (as it does on the non-game pages) as the day goes from dusk to dawn. It would really contribute to the atmosphere of the game.
I like the idea of Ur getting a little bit lighter or darker throughout a game day, but it may not blend well with some of the existing themes or areas. How would the meadow level with the bright rainbow look against a dark color in the evening? Will nocturnal Eggplants in the dark southwestern bogs die if the sky becomes a bright color at midday?