Try putting a solid colour, black or dark blue, above the day shot and reducing the colour's opacity to around 50%.

Day for night is a common color correction task. I'll summarize the steps from the "Color Correction for Digital Video" book.

With Color Corrector:
1) Desaturate to taste
2) Add some blue to the shadows and midtones
3) Drop gain and gamma
With Curves:
4) Crush some of the blacks on the blue curve
5) Clip the whites (of just the blue curve?--not sure)

I suspect you could find some tutorials out there as well.

I remember it's been discussed on this forum not so long ago and there was a few posts concerning lighting and a few post-prod tricks. Maybe do a search...?

To turn day into night start by dropping the track opacity down to about 40%. Then as Jason suggested lower saturation, gamma and gain. Depending on how dark you want it changing the blues in shadows and midtones isn't that much of a big deal. Use color curves, set a point near the middle, drag down and to the right so the center point is about a third of the way between its starting position and the bottom right hand corner. Now adjust the track opacity up or down as needed a little and you should be pretty close. Finish by tweaking gain and gamma so it doesn't look too harsh.

Here is the older thread futz referred to - all good suggestions.
http://www.sonicfoundry.com/forums/ShowMessage.asp?MessageID=137386&Page=0

Tiffen "Day for Night" filter on the front of your lens, same as the film guys do. Just be careful with framing, shadow and glare. I have it in 4 X 4 for the matte box but I am sure they'll have screw ons in 82mm, 72mm etc.

Peter

I've tried out the suggested effects and for the most part they're very good. I wouldn't say it presents a 'real' picture but its enough to suspend disbelief and create a useable night 'style'.

Its very good for backgrounds, and scenery. There really isn't one preset that can handle it flawlessly. Its really something that has to be considered and tweaked for each shot, even different angles of the same location.

But I'm after something much more. I want to maintain the colour of the people and darken whats behind them. Please share any ideas for doing it (or coming close to something approximating what I'm after). If this really needs a special program like AE or boris - I'd be interested in knowing what they offer.

For clarification - do you mean something like a still taken at night with flash? Where the flash only light up what's immediately in front of the camera?
Tor

Yes, thats a good picture of what I'm after.