I believe it's directly correlated to depth. The shallower, the lighter; the deeper, the darker.
As a follow-up, you can see from Google Maps (below) that the reason BOB has such astonishing beauty is because the lagoon--the part inside the islets--is
huge but in so many places shallow. That's one of the chief differences between BOB and MLE (the largest difference, of course, being that the former is blessed with Mt. Otemanu standing tall in the backdrop).
The map below shows the various gradients (i.e., the soupy mess of exquisite beauty), and I've noted in red boxes the places I've mentioned so far in the TR for your (and everyone else's reference):
The shuttle from BOB to Vaitape went south along the western side of the island, and then we took the cab from Vaitape all the way to the southern tip.
As you can see, the IHG LM has the distinct advantage of being able to see sunrise AND sunset, given it's location.