Color in Hubble images is used to highlight interesting features of the celestial object being studied. It is added to the separate black-and-white exposures that are combined to make the final image.
Creating color images out of the original black-and-white exposures is equal parts art and science.
We use color:
• To depict how an object might look to us if our eyes were as powerful as Hubble
• To visualize features of an object that would ordinarily be invisible to the human eye
• To bring out an object's subtle details.