[krunck]

Some lakes undergo drastic changes through the seasons. I know of a few that go from dark blue(late fall, early spring) to brown(spring runoff sediment) to dark green(algae in summer) in a single year.

[decimalenough]

The three lakes of Kelimutu in Indonesia are famous for changing color on a multi-year cycle.

https://en.wikipedia.org/wiki/Kelimutu