This is most probably a
Gradient Map. Here's a quick how-to:
- Open your source image
- Set your foreground and background colours in the toolbox: the
background colour should be lighter than the forground colour
- Add a
Gradient Map adjustment layer (
Gradient Map... or use the black/white circle icon in the
This will render your image in just the colours in a gradient of the two chosen colours, with the background colour matching the brightest parts of the image, and the foreground the darkest.
If you're not happy with the result, open the
Properties palette (
Properties), it will show you the gradient used and wil allow you to edit it by clicking on it. Since this is the standard gradient dialog, you can even pick a stock gradient or one you saved earlier.