User blog comment:JmLyan/Pixia, the Faerie Queen/@comment-6719710-20140812143001/@comment-7481207-20140812185454

1. Take an icon.

2. Make it black & white.

3. Create a new layer on top.

4. Fill the layer with a single color or a gradient of your choice.

5. Change the top layer's blending to "overlay".

And that's it. It really is simple once you know how.