From advanced aquarist online...
"With full realization that I'll get email stating that 'clams are not corals', I can not be content to leave it at that. So, I will make an even bolder statement - Tridacna clams' tissues (along with certain other genera) are colorless and contain no spectacular pigments. You are paying good money for an optical illusion. But what an illusion!"
thought that this is an interesting article.
http://www.advancedaquarist.com/2008/1/aafeature3