NOTE: this is an old article and is being revised. My apologies for the lack of photos.

Nebulae are distant clouds of gas and dust. We see pictures of them in glorious color, but is that what we'd we see with our own eyes if we were closer? If not, what color is a nebula?

We can see nebulae – they're grey
You can see the Orion nebula, which is between the two stars in Orion's sword. It looks grey. Through a telescope, you see the nebula magnified, but it's still grey. Apparently, if the telescope is large enough, some observers may see traces of green and red. But even if we were much closer to the nebula, we wouldn't see bright colors, because nebulae are very dim.

The problem is our eyes. They have two main types of cell that are sensitive to light: the rods and the cones. The rods respond at low light levels, but not to color. That's why objects seem washed-out in dim light. The cones are cells that let us see fine detail and color, but they need a certain minimum light level to respond. Nebulae, whether seen with the unaided eye or through a telescope, aren't bright enough to trigger our color vision. This is also true of a weak aurora. (You can find out more about this by following the link at the bottom of this article to “Star-gazing: Seeing in Dim Light”.)

The camera sees things that we can't
Light is a wave, part of the electromagnetic spectrum. Our eyes respond only to a small part of the spectrum. Optical astronomy is astronomy that studies the heavens in wavelengths that are visible to us.

Much of the light from a nebula would never be visible to us, except with special instruments, because it's outside the optical range. Although arctic caribou can see in ultraviolet light and pythons in infrared, humans can't.

Since nebulae are tenuous clouds of dust and gas, they aren't good light sources. Sometimes stars make them visible, but we still need a long exposure photograph to see them. A camera can do what our eyes can't, i.e., collect and store photons, building up a picture of a faint object. This might take several seconds, or in extreme cases, many hours. The Hubble Deep Field was imaged over a 10-day period in 1995. They used four filters with over thirty hours of exposure for each one.

The colors of diffuse nebulae
Diffuse nebulae are extended clouds without evident boundaries or structure. There are two types: reflection and emission. You can tell them apart in visible light by their color, and the color is the key to how they're illuminated.

A reflection nebula is blue. It's made visible by reflecting the light of a nearby star. The Witch Head Nebula is an example of a reflection nebula. Starlight is a mixture of colors, but the blue light has the shortest wavelength. It's comparable in size to the nebula's minuscule dust grains. This allows the dust to scatter the blue light more effectively than red light with its longer wavelength. A similar kind of scattering by the molecules in the atmosphere produces our blue skies.

Emission nebulae are red. They don't reflect starlight, they emit light at the atomic level. The strong ultraviolet radiation of a nearby star energizes atoms in the nebula, but the atoms don't stay in this high energy state. They soon give up the extra energy as light. Most of the gas in the nebula is hydrogen and it emits red light when shedding this extra energy. The Carina Nebula, an emission nebula, shows up red in a photograph taken using a large optical telescope.

The pretty pictures
That still doesn't explain all of the brilliant multi-colored pictures of nebulae from telescopes such as Hubble. Are they real? The quick answer to this is “no”, in that the colors in the pictures don't correspond to the wavelengths of light captured in them. They use a color palette in which important wavelengths are assigned certain colors. There will be a future article explaining how and why a palette is used, but for now here's an example.

You can see two versions of the Butterfly Nebula. The Hubble data files aren't paint-by-numbers instructions, so they can be processed in various ways. In this case, one image is the version released to the public. It's used the Hubble palette. In the other one a keen amateur astronomer matched the wavelengths of light in the data to the colors our eyes would see.

The “realistic” version is washed-out. The ultraviolet is prominent in this nebula and that's lost to us. In addition, some of the elements emit in shades of red that we don't see as distinct colors. Besides being prettier, the official photo - although artificially colored - could be considered hyper-realistic. It shows structure and distinguishes between elements in a way we couldn't otherwise see.

So, what color is a nebula?
If a color is what's represented by light in visible wavelengths, then nebulae have colors. The colors vary with the physical processes going on and the chemical elements involved, and we would need a timed camera exposure to see them. But that means wavelengths outside the optical range aren't colored. We can only see them if they're colored artificially.

Looking at those grey clouds, they really aren't much to look at. But with a little help from cameras and human imaging, we can see that, in fact, there is more to them than meets the eye.

Reference
https://www.astronomymark.com/hubble_palette.htm “The Hubble Palette”