I just learned what a duotone effect is and how to create it in Photoshop. It was so exciting that I immediately tried it out on the above photo of Tower Bridge.
First I applied a gradient mask, which created a monotone (or black-and-white) version of the color photo. This made the darker areas dark gray/black and the lighter areas light gray/white.
Then I changed the colors of the gradient to fade from dark purple to light green, instead of black to white. And that was it! Kinda cool, I thought.