reflectionvisible-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.
This is an example of fluorescence. Usually, when you shine a laser beam on a material and the photon is absorbed by an atom or molecule, a photon of the same frequency will be re-emitted. Sometimes, though, when a photon is absorbed, exciting an electron to a higher state, the electron doesn't decay to the ground state right away, but in several steps. In this case, when you shine light of one frequency on the material, it may emit light of a lower frequency. This is called fluorescence. Apparently whatever dye is using to color the orange plastic fluoresces orange when you shine green light of the right frequency on it.
There are lots of materials which fluoresce in the visible spectrum when UV light is shined on them. You don't see fluorescence with red lasers, because red is the lowest frequency of visible light, and photons emitted by fluorescence are almost always lower in frequency than the ones absorbed (there are apparently a few exceptions; see the wikipedia article). But it should be possible to see red, orange, or maybe yellow fluorescence with green lasers.
And here is a discussion of this phenomenon (with videos) on a blog.
One half of the problem is light. Light is an oscillating electromagnetic field. The frequency of oscillation determines the color. Higher frequencies have shorter wavelengths and are blue. Lower frequencies have longer wavelengths are red.
Light is a mix of a range of wavelengths. Sunlight contains a wide range of wavelengths, including some that are too long or short for us to see. A laser contains a very narrow range.
How light interacts with matter is the other half of the problem. Electromagnetic fields exert forces on electrons. Electrons in atoms or molecules are constrained in how they can move.
Metals have electrons that can move freely. This makes them good conductors. Freely moving electrons reflect light. Light makes electrons vibrate and absorb the light. Vibrating free electrons emit light and stop vibrating. Thus metals are shiny.
But even in metals, electrons are constrained. For example in copper, electrons can vibrate more readily at low frequencies. Copper reflects red light better than blue. It reflects better still in the infrared and even worse in the ultraviolet. Here is a graph from a random website, http://www.minoanatlantis.com/Minoan_Mirror_Web.php
So if you shine a red laser on copper, it reflects almost all of it. If you shine a green laser on copper, it would reflect less.
Other materials have different ways of interacting with light. But at each wavelength, they may reflect some and absorb some. They reflect more at some wavelengths and less at others. Even black materials reflect some light.
Best Answer
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."