Night time scenes are hard to light because at really low levels the 'dark' colors are still too colorful.
This nodetree takes the brightness of a pixel, removes the color management (Filmic usually), does a correction for the RGB response of the eye, thens puts the Filmic back. It uses the resulting value to mix the full color version and a BW version. Bright pixels stay colorful, shadowy areas are color muted. There is one setting to control the amount.
The included picture is an example from a project. I brightened the sample image so that the desaturation was easier to see.