A gray day at the lake

Hi Everybody,

   It was a gray day when we took this shot.  I tried to cheer things up  by shifting the overall color to blue by using the b Lab curve.  It became a lot more pleasing.

   This is another mystery that I want to solve before I go upstairs.  How do the Lab curves maintain color and lightness when a color is increased or decreased.  If you decrease blue with the RGB curves, the image turns to yellow and gets brighter. In the Lab Curves you can put a node in the center of the b curve and then increase or decrease blue without any change in hue or lightness,

    What magic is being used.

Phil