Synthetic RGB from [SII], Halpha and [OIII] Emission Line Data 

[SII] = Red, Halpha = Green, [OIII] = Blue       click to enlarge 

click to enlarge

Synthetic RGB: [SII] + Halpha = Red, [OIII] = Green, Hbeta + [OIII] = Blue, scaled Halpha = Hbeta

Click to enlarge

The key thing that affects the color balance of most emission nebulae is the intensity of the Halpha, the [OIII] and Hbeta lines. In particular the ratio of Halpha to Hbeta is critical to get the color balance right. 

When blending in Halpha into an RGB image or when using Ha and [OIII] to make a pseudo RGB image, it is important to put a synthetic Hbeta line in the Blue channel to make the color balance reflect the spectral energy distribution.

The naturally occurring ratio of Halpha to Hbeta is approximately 2.92 to one. There can be extinction of the higher energy species, Hbeta, so the effective ratio observed on Earth will be somewhat skewed more in favor of Halpha typically.

In order to get a reasonably realistic balance of color try blending about 15% to 30% of the Halpha into the [OIII] and use that for the Blue channel.

If you happen to have [SII] data too, then you can blend a scaled version of the [SII] with the Halpha to use for the Red channel, since [SII] and Halpha are both in the Red part of the visible Spectrum.

Click chart above to read source paper: http://articles.adsabs.harvard.edu//full/1994JBAA..104..231B/0000231.000.html

here are images processed as synthetic RGB using Ha and [OIII] only and using the synthetic Hbeta method:

Ha = Red, [OIII] = Green, [OIII] + X * Halpha = Blue

Ha = Red, [OIII] = Green, [OIII] + X * Halpha = Blue

  Ha = Red, [OIII] = Green, [OIII] + X * Halpha = Blue

  Ha = Red, [OIII] = Green, [OIII] + X * Halpha = Blue

The image of M33 below includes [SII] in order to make a pseudo true color image but one that accentuates the HII regions

Ha + X * [SII] = Red, [OIII] = Green, [OIII] + Y * Halpha = Blue

Here's a comment from the University of Wisconsin Halpha Mapper website:

http://www.astro.wisc.edu/wham/science.html

"Since most of the H-Alpha emission we detect arises from hydrogen recombination, atomic physics is the only thing that dictates the ratio of H-Alpha to H-Beta emission from most ionized intersteller (sic) gases. Although it is a slight function of temperature, near 10,000 K, the ratio is about 3:1 in favor of H-Alpha. However, intersteller (sic) dust absorbs more blue light than red so that ratios greater than this are typical in observations. Observed ratios of H-Alpha/H-Beta should be an intersesting (sic) probe of dust in front of and within the ionized gas."