A surface filters the light reaching your eyes
An object does not send its own light into a room; it redirects some of the light that strikes it and absorbs or transmits the rest. A red-painted surface absorbs much of the incoming light while reflecting more of the wavelengths that people typically perceive as red. Under broad-spectrum white light, those reflected wavelengths reach the eye, and the brain interprets the resulting signals as red.
The eye does not measure wavelengths one by one. Three kinds of cone cells respond to overlapping ranges of light, and the brain compares their responses to produce color perception. The light arriving from a surface therefore depends on two things: which wavelengths the lamp supplies and which wavelengths the surface reflects.
Why the same object can look different
Change the lamp, and you change the ingredients available to be reflected. A red material under a blue-only light may absorb most of that light, leaving little energy to reach the eye. It can look very dark, even though the material has not changed. Under a different lamp, its appearance may shift again because the reflected mix stimulates the cones differently.
This is why color matching can fail between settings: two samples that look alike in daylight may look unlike each other under a narrow-spectrum lamp. Color is a relationship between illumination, a surface’s reflectance, and the visual system—not a fixed glow stored inside the object. A red sign can keep its red pigment in a blue-lit room, but without suitable incoming light, it cannot return much red light to the viewer.
