The bow looks to be palm, I agree. Not sure about the species of palm, as there are many species which can look similar. It could be black palm, but I've seen much darker 'black palm' as well.
The lighter colored side is the interior section of the palm 'tree', while the darker colored side was the exterior face. The lighter colored side should have less fibers and as a consequence is lighter in weight and strength as well. I suspect this lighter side to be the back of the bow; grasses such as bamboo and palm are much stronger in tension than in compression, so the additional strength from the fibers is more needed on the belly.
Please don't take my guess for the truth, but if I had to put my money on it, I'd say the darker side is the belly.