I think it's the color of the substrate. I temporarily moved my seng from a tank with dark gravel to a tank with light gravel yesterday. He was noticeably lighter in color and I could see the black specks along his back better.
I know what you're talking about! All my round face bichirs had them, even the ornate. It almost seems as if the green markings would react to a UV light. I asked a long time ago, and no one seems to care or notice, I figured since all of the round faced bichirs I kept had them, it was normal. Also, the markings were still present in my senegalus bichirs past 8", not fading with age.