This composite image of Kelimutu volcano in Indonesia was the NASA Earth Observatory Image of the Day for July 6, 2018. It called them volcanic mood rings and explained:
From milky white to vibrant turquoise to blood red, the three lakes at the summit of the Kelimutu volcano are known to unpredictably change color – a phenomenon unique to this volcano on the Indonesian island of Flores.
These images, acquired by the Operational Land Imager on Landsat 8, show the various colors of the crater lakes on three different days. All three crater lakes appear on the crest of the volcano with the eastern two lakes sharing a common crater wall … Depending on when you visit, the colors can range from white, green, blue, brown or black. In 2016, the lakes changed colors six times.
Local folklore contends the lakes are the resting place for the dead, and that a person’s good or bad deeds in life determine which of the three lakes becomes his or her resting place.
Science says the changing colors of Kelimutu’s summit lakes are caused by fumaroles, or volcanic vents that release steam and gases, producing upwelling in the lakes and bringing denser, mineral-rich water from their bottoms to their surfaces. NASA Earth Observatory explained:
All of the lakes contain relatively high concentrations of zinc and lead.
While minerals play a part in the coloring, another key factor is the amount of oxygen present in the water. Like your blood, these lake waters appear bluer (or greener) when low in oxygen. When they are oxygen-rich, they appear blood red or even cola black.
Bottom line: NASA Earth Observatory Image of the Day for July 6, 2018, showing the three variously colored lakes at the summit of Kelimutu volcano in Indonesia.