It adjusts the red / green / blue in different part of the photo.
The line indicates different part of the photo. The upper, the brigher, the lower the darker. You raise / decrease the line to add / remove the color.
3 options: red, green, blue.
Click the red to adjust the redness of the photo:
Raise the upper part of the line, you add red to the bright part of the photo.
Decrease the lower part of the line, you remove red from the dark part of the photo.
By this way, you can adjust the green / blue in your photo.