Why would your praying mantis be turning brown and hollow?

It really depends on the species. Some mantises of the same species may be green or brown, depending upon the climate and surroundings.

If the area is humid or wet, they'll most likely be green.

If the area is hot and dry, they'll most likely be brown.

If their immediate surroundings are somewhere in between, they may be partially brown and partially green! Go figure!