i want to offer a few corrections to this post: it wasn't the Triassic, it was the Protoerozoic. not 250 million years ago, but 2.4 billion years ago. and it wasn't pink before, then red, it was both at the same time. the pink came from cyanobacteria, and the red came from the iron oxide that flooded the sea during The Great Oxygen Catastrophe.
The surface-dwelling cyanobacteria flooded the earth with Oxygen through en-masse photosynthesis, and then died, because they couldn't live in the highly oxygenated environment they created. the oxygen would seep into the oceans and bond with iron at the bottom, forming iron oxide.
this is what turned the seas red, and also what created these gorgeous banded iron formations (first discovered in northern Michigan in 1844, they make up about 60% of all iron reserves)
If you wanna know more, check out this video