I know some monitors have a mono/green switch, but I'd like to do this with my 1702. If one doesn't exist, I'll probably try and make one (somehow).
I don't know if such device exists as a final product, but here's some hints about how to build one:
Composite video signal is made up from the analog sum of two components: luminance and chrominance. We are interested in the luminance component. So you should rip off the chrominance signal by using a low-pass filter with the cut frequency set at 3.5MHz (for NTSC signals) or 4.43MHz (for PAL signals). If no high resolution graphics are going to be displayed (that is, no more than, say 320 pixels per scanline), you may go with a low-pass filter with a cut frequency of 3MHz which will work on both PAL and NTSC systems. Analog Devices offers an online filter generation tool here: http://www.analog.com/designtools/en/filterwizard/ which I have used to generate this filter (I'm not by any means an expert on analog electronics, so this filter may totally fail)
The resulting signal is monochrome video. To make it amber or green you need to get the video and sync information on separate signals. To get syncs, you may use an LM1881 video sync separator chip ( http://www.ti.com/lit/ds/symlink/lm1881.pdf) , which will give you an output (pin 1) with sync signals only.
To get the active video signal, use a sync stripper as the one described in Intersil AN9752: http://www.intersil.com/content/dam/Intersil/documents/an97/an9752.pdf
Now you have a monochrome video signal and a composite sync signal. You can use this video signal as the green channel, keeping R and B channels black (tied to ground), to get a green over black image. For amber, you can route the video signal to the R channel, and using a voltage divider, route half this video signal to the G channel, keeping the B channel black, to get an ambar-like colour.
This way, you will get an RGB signal with the configuration you want. If you need a composite video signal as final output, use an AD724 PAL/NTSC encoder ( http://www.analog.com/media/en/technical-documentation/data-sheets/AD724.pdf ) to get a composite video output from the composite sync signal provided by the LM1881, and the RGB channels, arranged the way you like from the monochrome video signal.
If you are going to do this "properly", @mcleod_ideafix's pipeline of circuits to extract the luminance and then re-encode it onto just the green channel is the way forwards. I would enhance the design by adding some sliders/pots to allow tuning the exact RGB colour, since "green" monitors and colour monitors did not always use the same shade of green phosphor, and also you might fancy an amber display one day.
However, the way I'd probably do this, at least as a prototype, is to turn the "colour" right down to get a monochrome display and then stick some coloured film over the screen. Hey, it worked for the original Space Invaders! It would probably also have fewer transcoding artefacts than multiple stages of re-encoding a composite video signal.