Rio 2016's organisers have admitted that more "could and should" have been done to test the Olympic diving venue after the water turned green yesterday.
The open-air Maria Lenk Aquatic Centre, which was built for the 2007 Pan American Games, is hosting the diving, synchronised swimming and water polo events.
Staff at the venue originally said the diving pool's emerald shade was a result of harmless algae caused by the heat and lack of wind.
But today, swimming's world governing body FINA revealed that the long duration of an Olympic diving competition led to the water tanks running out of "some of the chemicals used in the water treatment process".
FINA stressed that there was no health risk to the divers but said "the pH level of the water was outside the usual range, causing the discolouration".
Rio 2016's spokesman Mario Andrada said: "We did have test events with the same number of divers but we are using the pool for a longer period now.
"The people in charge could and should have done more extensive tests during the day - we probably failed to notice what would happen over time.
"There is no risk to the athletes - an independent group has confirmed that - and the pool should go back to the classic blue colour soon.
"Today's rain has affected that a bit, but we are testing the water quality regularly."
Pictures from the venue on Wednesday show the diving pool is still tinged with green, while the adjacent pool used for the synchronised swimming and water polo appears to be going the same way.
The divers, however, do not seem to be remotely concerned, with several saying the change has actually helped them.
Britain's Tom Daley, who won a bronze medal in the 10m synchronised diving on Monday, told Press Association Sport: "It's slightly strange to look at...but it actually does make it easier for us to dive because normally, when you're spinning, it's difficult to see which way is up when the sky and water are both blue.
"So when it's green, it does make it slightly easier. It's nice and warm, too."
His diving partner Dan Goodfellow agreed, saying: "We've been told by the doctors it's safe, so I'm sure it won't affect any of the diving events.
"Water is water, isn't it? So it's not going to affect our dives. The main thing is it won't make us ill."
Canadian synchronised diver Meaghan Benfeito was another to call the situation "weird" and said the stronger contrast with the sky helped her, too.
But Benfeito admitted she and her partner were trying not to laugh about it while climbing the diving tower and exchanged some last-second advice: "We said, 'Don't open your mouth in the water'. Just in case."