Round About Midnight

Step 1:

1. Take one fairly mediocre image
2. Posterise down to 2 bits per channel
3. Split into RGB channels, and pick the one with most detail (red in our case)

   What we've done is reduce out starting picture down to a very colours, in fact 4 (2 bits means four colours for each Red, Green, Blue channel. When we get down to only a single channel there are only 4 colours left.

   We are going to use this simple pallete picture as a moonlight overlay.

Step 2:

1. Now, using the colour replacer we want to replace the various shades of grey with mostly blues, but a little light pinks and light greys too.
2. Before you can do this, you will need to set the image to 16M colours. Each of the RGB channels has only a 256 colour (grey in fact) palette.
3. Colour replacer might not quite go far enough, I've also tweaked up this final image by hand with paint brush.

Step 3:

1. Return to your original image.
2. Typically, the sky will be bright, not what you expect on a cloudy night!
3. With the magic wand, select the sky and darken it. Don't go to flat black, leave a little texture in.

Step 4:

1. Now, copy hand crafted channel image (Ctrl-C) and paste it over the original image as a new layer (Ctrl-L).
2. Set the layer blend mode to Multiply

Step 5:

1. Finally, you might want to go back to the original image and reduce the saturation
2. Select the lower image
3. Use Colours, Adjust, Hue/Saturation/Lightness (Shift-H)
4. Keep Hue and Lightness on zero, and pull down the saturation. Saturation is basically the amount of colour in the information. Because of the way our eyes work we see dark nights with less saturation.
5. While you are here, you might also want to adjust the lightness.

NOTES:

On an image this size, the effect is frankly pretty crude. You need to work on a larger image to achieve the best effects.

Firstly, although we've used the split channels function here, it is only to set ourselves up with a low number of colours within an image. This could have been done using straightfoward colour reduction, or switching to greyscale and reducing.

Secondly, remember one of the functions of night is that it night sight is under saturated, so pull down on the saturation.

Thirdly, night vision and moonlight tends to push towards the blues and silvers, use those colours, but a little pink can warm and provide contrast.

Fourthly, there are no hard and fast rules here. Whatever changes you need to make will be driven by your original image and by where you want to get to. Experiment with this technique.