Why are corals colorful?

So my question really isn't what proteins or zooxanthellae or physics is involved in producing the colors. I'm more curious as to why they developed the proteins and zooxanthellae to take advantage of the physics of light to produce the dazzling colors?

Stepho brought up an interesting explanation, that the bright colors serve as a sort of advertisement to other species that live in the reef community.

 

So why am I looking at a favia in my tank that has both brown and green? Why does each polyp on my zoa have 2 to 3 colors? Why does one of my open brains have a green center with a bright red perimiter.? If it was based on light and depth you would expect all corals at a certain depth to be the same color and to be a solid color. (I'm not trying to argue by the way. Just asking questions.)

 

That would be an interesting read when it's all done. I hope you do well.

 

without making this too nerdy-like, i would say that our corals exhibit different colors at these specific places because there are different cells that take out different metabolic processes in the coral. For instance, the green in your favia is around the mouth, this particular type of zooxanthellae also has a UV photophosfluorescent pigment that is excited while the outter brown zooxanthellae part does not. Im taking an educated guess here but since it is around the mouth i think that it is to provide more energy to the direct area (aka feeder tentacles). Now if you look at the new growth on this favia, you will see that the new skin is clear until some zooxanthellae can host it.

I can go deeper if you want but it will be a little more technical... you can pm me if you have more :]