• Create an inverted selection of the blue channel (cmd+click on the Blue channel in the Channel palette) and select the inverse by pressing cmd+shift+i.
  • Make a new curves layer. That inverted selection is automatically turned into the mask for our new curves layer.
  • Change the blending mode for that layer from “Normal” to “Luminosity”. This stops the colors from shifting but will change their brightness.
  • Add two points to the curve, pulling the right up and the left down, creating an S shaped curve.