Since you're boy hasn't changed color on you it's more likely that he's a multi-color instead of a marble. PHILOCTETES - If the green only appears under certain lighting then it's sounds mostly like irridecence (see below).
You may have seen these already but here are 3 good links on betta color (written for breeders however so a bit of a focus on genetics).
Betta-territory color's article
BettySplendens color archive Bettas by Jim Sonnier (Colors/Genetics)
Irridecence vs Metallic


Here is my boy Coolie. If you compare the two pictures you can see that he doesn't appear quite the same color. In the first picture you're seeing his irridencence. It's the top most layer of color in betta's and gives them the shimmery effect.
Metallic is a different gene that effects the same layer and was introduced into the betta hobby by outcrossing Betta Splenden with wild betta species (speciffically betta imbellis, smaragdina and mahachai due to their naturally higher degree of irridecence).
"This logically brings us to the next question: as these hybrids are not new in our hobby, why did it take such a long time to develop the copper and metallic bettas as we know them today? It is not easy to answer this question but probably the betta community earlier was not really interested or ready to breed these varieties and they did not recognize their potential. Around 2002-2003, the Asian breeders offered the first copper imbellis types on aquabid. These fish were created by selective breeding and stood at the base of the worldwide copper and metallic fever as we know it today.
By outcrossing these fish to longfinned lines it did not take long untill the first copper imbellis deltas arose in the betta-scene. Outcrossing these fish to iridescent (turquoise, steel blue and royal blue) halfmoons produced the first metallic blues [10]. The offspring of these types of crosses still contained a lot of red wash. Continuous selection for both color and finnage finally
LED to longfinned halfmoon metallics without red wash.
Outcrossing copper phenotypes to pure iridescent turquoise, steel blue and royal blue fish added another new trait into our hobby: the mask feature. Normal iridescent bettas are characterized by their typical black heads. Outcrossing to copper bettas led to an increase color spread into the face and head area of the fish. By now it is known that the metallic and mask trait are caused by seperate genes and can work independently." *quoted from article below*
What the specific differences are between the two genes I can't properly explain. However the IBC's Magazine "Flare" had a really good article written up about it (
This article was published in Flare (Journal of the IBC) - May/June 2008, Volume 41, No. 6. )
Here's a really good page to see to compare the appearence of Irridecence, Metallic, and Dragon type betta coloring.
Hope that was somewhat helpful,
