Fuego has a shell painted to look like a lady bug's shell. Houdini's original shell is painted orange with colorful swirly patterns. I guess it was out of boredom they decided to exchange properties or something.
It was an amusing surprise. The lady who sold me those hermit crabs did tell me about this habit. Fuego and Houdini have been with me since November (or was October?) and they haven't exchanged shells until now.