But the colors we see in these photos ARE actual colors being emitted by the objects (assuming it’s an optical light image, as opposed to infrared, ultra-violet, microwave, gamma, x-ray, etc, none of which we can ‘see’ with our eyes of course), it’s just that our eyes are most sensitive to the greenish gray color you referred to in that first Ring Nebula image. In order to see these other sorts of images, the various invisible-to-us wavelengths need to be converted into visible frequencies, or ‘colors’.
They are the real colors, but I chose this to illustrate the methods used to add false colors. Black and white images filtered for specific infrared, ultraviolet, or x-ray frequencies are falsely colored and layered the same way.