how many of the Hubble photos posted hereon are natural colors? None. The colors have to be reconstructed from multispectral data. Come to think of it, this could also be said of color photography. Natural colors must be seen directly by the eye, not by looking at some artist's impression.
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.