The areas that become bluer on older Croceas are similar to the rainbow color pattern that are seen through water that may just be laying on the ground. It's just like refraction to an extent. But the Iridophores on the mantle are blue and group together for a reason. They form other areas called schemochromes that direct light into the iridophores which is where the majority of the zooxanthellae tend to "gather".
Here's an easy way to figure it out. If you want - Look at a close up of a crocea mantle and try to count all of the little bumps from the full eyes all the way down to the pin sized ones. Then look at a Gigas, Maxima, Squamosa, Derasa and count them (if you want - to me it's fun). In 99% of the cases, there are going to be hundreds if not thousands more around a croceas' mantle. Granted maximas are not that far off, but their coloration patterns tend to direct light easier. Croceas are just "made" for certain depths with certain lighting. I've just been doing A LOT of work with them lately. And am hopefully heading down to the Keys to do more.
I have even been wondering if I breed Croceas and innoculate them with a less light intensive zooxanthellae, if that will have any effect on their long term survival.