Emerald Mithrax crab question

[moogoomoogoo]

Do they have to be green? I found 2 hitchhikers on the live rock. The crabs look just like the pictures, except they are bright white.
Albinos? Normal color variation? Wrong species? Sorry I haven't figured out how to get my camera's photo to this web site.

 

[dc]

I've heard that the green ones with red in their joints were the only 'safe' ones. And that's open for debate, as a lot of people do have trouble with them also.