I don't think it's a secret that Bongorob and I saw one of the specimens held off show at Chester, on our ZooChat tour. That one had yellow markings. The three I saw on display at Regents Park also had yellow markings. The Boelen's python webpage shows photos of wild individuals with both colours Description of Natural History and Geography By Marc A. Spataro
Chester has now put two specimens on display. I haven't managed a decent photo yet because the most spectacular feature is the iridescent sheen on the black scales of the body.