Hi, that's a tough one to give a simple answer to but take a look at some photos of buildings and you'll see that you have a number of options....
They could be light grey, maybe they have an overhang that casts a shadow over them, so they would be darker in certain areas at certain times of day...maybe they're tinted, so they would be even darker still...maybe they reflect the sky or the landscape around them.
It's not a weird question at all, it's just one that doesn't have a one word answer. Hope this helps.
Ed
