What colour would a green object be with a green light shining on it?

A green object under green light would appear green, as the light matches the object's color. The green light illuminates the object, causing it to reflect the same wavelength of light that it absorbs, resulting in no change in its perceived color. If the light were a different color, the object's appearance would change based on the light's wavelength and the object's ability to reflect or absorb that light.