Convert the mode of your image to LAB. (Image/Mode/Lab Color)

Open the curves dialogue (Image/Adjustments/Curves) In the channel pop-up menu at the top of the curves dialogue, choose “a” and then drag the top right curve point two or three squares to the left, and then drag the bottom left curve point two or three squares to the right. Do not click “OK.”

4. Next choose the “b” from the channel pop-up menu and (as before) drag the top right curve point two or three squares to the left, and then drag the bottom left curve point two or three squares to the right. Now you can click “OK.’” Convert back to RGB