[jdboyd]

I believe the contrast adjustment was likely to get rid of any unwanted shades of grey before colouring the two extremes.

[jameshart]

I suspect 'coloring the two extremes' involves something more like 'wiring the luminance signal up to the chroma'. Again, try not to think digital. This isn't photoshop, this is signal processing.

[tomc1985]

Or it could be a filter over the camera lens.

[Jaruzel]

Once you have a mostly Black, and White (with no greys) image - it's relatively simple via basic electronics to colour replace those with alternate colours - much like the old analogue chromakey 'blue-screen' techniques.