Reduce your photo to two colors. You can define the change between dark and light colors continuously. The both colors can also be defined individually.
This effect works simila to the threshold effect. But instead of replacing colors all colors above the threshold will be turned into black and white.
Use the lightness slider to adjust the transition between the colored and the black and white areas.
Use this effect to color parts in your photo, that have a certain lightness.
All colors which are in a lightness range (use threshold) are colored with your favourite color. Use expansion to expand or shrink this areas.