Not really seven — but at noon, you see them all.
The lagoon isn't seven colors. It's a name. But at midday, from the dock or from a kayak in the middle, you see why nobody corrected it.
Three things do the work: depth, what's at the bottom, and the angle of the sun. Pale gypsum in the shallows reads as a light turquoise — the kind that looks edited and isn't. Cenotes go to a navy so deep it's almost black. Between them, every blue and green.
Midday is when all of it shows. The sun is high and straight, reaching the bottom of the shallow parts and stopping cold at the edge of the cenotes. That contrast is the count.
Early and late have their own reasons — stiller water, better photos. But the color itself is a noon thing.