Can I substitute brown or confectioner's sugar for granulated sugar in sugar cookies?
I've done this before, and you won't get exactly the same cookie as you got before.

Confectioner's sugar is a total bomb. Don't bother.

Dark brown sugar makes darker cookies, with a chewier texture, which keeps longer.

This isn't always a bad thing. Several of my cookie recipes I thought were a bit dry were saved by DBS. It added a nice depth to the flavor, as well.