[sig226]

Almost all of them are enhanced or manipulated. Even if their only manipulation is to make a long exposure, the image produced is no longer what the human eye can see.
This is what you would see if you looked through a telescope at the Ring Nebula, M57.

A composite image is made of several exposures stacked on top of each other. It enhances the colors.

Hubble image of the same thing:

This was created using different black and white exposures which were filtered to collect light in different colors. The colors were replaced when the image was assembled.

[Quix]

Impressive stuff.

tHanks.

[ETL]

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’.