What colour would a green object appear in orange light? What colour will a green object appear when it has an orange light shining on it?
As far as I can work out if the light source is from e.g. a TV or monitor using the RGB system the orange light would be a mix of red and green (with more red) and so the green surface would absorb the red and reflect the green so the object would appear dull green.
However I want to know what would happen if the orange light source is not a mix of other lights but pure orange from the electromagnetic spectrum.
 A: If your subject is illuminated only by a single pure wavelength of light, then all you will be able to see will be different shades of one color. If the light is "orange," and the subject is "green," then the subject probably will appear to be very dark, maybe black depending on how much of that exact wavelength it reflects.
If you live in a country where low-pressure sodium vapor lamps are commonly used for outdoor lighting, then you can easily see it for yourself.  Those lamps produce all their light at two, very closely spaced, single wavelengths. They look yellow, and everything you see under low-pressure sodium lighting will appear as shades of yellow.
Note that the orangeish-pinkish high-pressure sodium lamps commonly used in the U.S.A. are different because the arc puts out multiple wavelengths, and the ceramic capsule that contains the arc gets white-hot (i.e., produces significant incandescent light).

NOTE though: If you look at a subject illuminated by one single short wavelength, then some minerals and some man-made dyes will fluoresce. That is to say, they will emit light with different colors than the color of the light source. There are just a few dyes that will fluoresce under green light, more that will fluoresce under blue light, and quite a few that will glow under so-called "black light."
