Some objects seem to have the same color whether the light that we perceive is emitted or reflected Is color only a property of perception, considering these two examples:


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*The glass used in a green traffic light looks green no matter how it is illuminated, either by a white light bulb behind it or a white light bulb in front and

*A green LED (to stay with the same color) emits a green color and receptors in our eyes that are excited by green light are stimulated and we eventually perceive this emitted light as green. Does not the correspondence between the color of the objects(LEDs) and our perception indicate that some property of color is a part of the "real" world? 
 A: "Color" is a property of perception. "Optical spectrum" is a physical property of light and objects that interact with it. There is no great mystery behind the (scientific) definitions of the words "color" and "spectrum", which refer to very different aspects of light.  
When a physicist talks about "red light", it only means that the choice of light source is either obvious or simply not important to the experiment. If the light "looks reasonably red" to a person of normal eyesight, it will probably do in the context the physicist is talking in. 
However, if you want to explore the detailed relationship between color perception and physical spectrum, then you will hit on a large number of important and non-trivial relationships that are partly a consequence of the physics of light and partly a consequence of biology. I would say that even a cursory exploration of the topic is way past what can be done on this site. 
A: The light that you see is usually a mixture of photons of many different wavelengths. These photons interact with the cones in your retina, and based on the relative perceived intensities, the brain draws a conclusion about the "color" of the light. This is a PERCEPTION - but it is closely related to the mixture of photons that hits the eye (although it is possible for different mixtures of photons to "look" the same)
This has NOTHING to do with the source of the light - how it was generated, and what process selected photons of some color to reach your eyes while others did not.
In general, the following processes give light "color":


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*Bias towards certain wavelengths over others at the time of generation: think lasers (quasi monochromatic), sodium lamps (specific atomic transitions are favored), black body radiation (more red shifted at lower temperatures), etc.

*Scatter of certain wavelengths: the sky is blue because blue light is more likely to be scattered by the atmosphere (so the blue component of sunlight is scattered towards your eyes), and the setting sun is red (because the sunlight traveled through a lot of atmosphere to get to you, and the blue components have been scattered out of the direct beam)

*Absorption of certain wavelengths: the red cover of your tail lights absorbs blue and green components of light more readily, leaving only red light to escape. Similarly for the green cover of a traffic light (more below)

*Reflection of certain wavelengths: when materials are hit with different wavelengths of light, this light will typically penetrate some distance into the material and scatter. After several interactions it will "escape" again - but in the process it is likely that some colors were once again absorbed. This gives many materials their surface color.

*Interference: when you look at the coating of some lenses, you will see a color. This can be due to interference effects (some wavelengths experience constructive interference). Oil films on water can do something similar

*Refraction: a rainbow is created when white light is refracted through different angles depending on its wavelength. As a result, the light from the sun reflects differently at different colors, and the white light appears spread out.


This is not a complete list - but my point is that there are many physical mechanisms for selecting the color of the light that reaches your eyes.
As for the fact that the cover of the traffic light looks green whether light is shining through it, or whether light is reflecting off it - this relates to the mechanism that gives the material its color. The color you get in reflection can depend on the material properties. If you have "really intense" color (think - dried up ink from a fountain pen) you will sometimes see that the reflected color is (almost) the inverse of the color you get at lower concentrations: atoms / molecules near the surface resonate at certain frequencies and reflect these back immediately. When you have colored glass or other "less intense" colors, the light tends to penetrate some distance; at this point, the resonances tend to result in absorption and the color you see in "reflection" is the same as the color you see in "transmission". This is the case for the traffic light.
I hope the above sheds some light on the topic. If it does not, then you should get some rose tinted glasses...
Further reading: http://www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission
