I tried aspen in my cages and discovered that it was difficult to see when they had peed a small amount and ridiculous to clean when they had pee'd a large amount. I truly felt that I was not able to keep their environment clean.
I use indented kraft paper that I buy from u-line and have delivered. It is a light brown paper and so I don't mind the look of it.
I have a bunch of aspen left that I've sealed in a bag and will probably use for my boas when they grow large.