It is usually light brown to almost black, but I would wager it highly depends on the food they eat. It's usually oval shaped and they seem to release a single stool.

They do product solid waste if you want to spend time you could observe them (not a really productive way to spend time unless you think your frog is constipated), they do poop eventually.

I think once they defecate though the poop simply starts to break down and is sucked into the filter, eventually.