Air-Valve Autobot!
How does underpainting work ? Do you paint the entire picture in mono grey and then just go over with transparent colours
I've seen them used two ways, but I'm sure there are a few more techniques out there.

Values of the painting are done in light layers of blackish color, then overlaid with transparents.
The only problem I see with this is being able to achieve realistic shadows with colors like yellow(It tends to look like a dirty green)

Glazing over Opaques: (Glazing what using transparents is called in the hairy brush world)
Opaque paints can be mixed to match color, but end result can look flat and dull.
Transparent paints(even yellow) darken the value of what they are being sprayed over.
Match your opaque colors to the hue, chroma, and value of your target color, and use them for your underpainting. Add a little white to get the value of the color about 1 lower(on a scale of 1-9).
Use transparents to do the final tinting and color shifting. The transparents being overlaid will make the opaque colors darken in value and increase vibrancy.

Marissa is the master at this stuff, see the picture below. It looks like she is using a form of the "Glazing" method(not a real method name, just ad libbing)
Marissa Sample.jpg

Image Source: http://marissaoosterlee.com
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Thanks jagardon.. very interesting way above my skill capability at the moment but something I would like to try at some point
If you use a dark colour just for your shadow underpainting and then use transparent paint over the top you will be able to use the underpainting to create your shadow tones but you will need to use some opaques to build up the areas before the transparents. As for doing the whole painting in monotone then yes it can be done but its more of an old masters oil based technique but you can still do it using acryllics. Check out " a portrait of Anna " that is an oil painting with 2 monochrome layers. very good to see how its done though.

I have also come across the term "underpainting" when a person first creates the painting with a light amount of the complementry color and then adds on top the main color. I have tried this a lot with water colors (not airbrushed... my skills are not at such a level yet) with good effect. With the right care and forethought the colors are very rich (some times too rich).