The main lake appears to have been edited into this picture. It's way darker and has no transition between the closer water and the lake. Far shore is way to even to be natural
What Carlos explained in his native tongue was this - "An artificial lake resulting from the washing of the old ore mines of wolfram in the National Park of Peneda-Gerês/Portugal." I think the difference between background lake and foreground lake is less one of colour than texture, the visual difference may be simply due to the effect of the change in texture, a kind of visual trick.
Joanie