Day for Night Example
Original Photoshop Image After Night "Conversion"

This is from the Photoshop sample folder.
The left photo is straight except for conversion to 8-bit color. The right side
is the same photo with three changes:

--I deleted 75% of the color (Image, Adjust, Saturation).
--I pushed the middle of the gamma curve down (Image, Adjust, Curves).
--I pushed in the slighest amount of blue.

If it looks good without the blue step, leave it out.
You can tune all these steps as you want for best effect.