This should work with any version of GIMP, I think. And perhaps even with other image editors that can use the same kind of layer mode.
1. Open an RGB image.
2. Add a layer above the base layer, color it 80,80,80 (middle gray).
3. Set this new layer's mode to Grain Extract.
4. Now for the artist in you: Use Colorize to color the gray layer and thus affect the color of your image.
Hue determines which color to add to the image.
Saturation affects by how much (0 = original image).
Lightness, well, try it out.
There are endless variations you can affect your image. From warmer via more yellow or greener, to colder looking using some blue tint.
I thought this was slightly easier to do than using other tools, and sometimes many of them, especially since I wanted a very heavy effect because I was doing a painting-like editing.
