PP4 currently just uses plain red for HII regions.

Calculating a Better HII Color

Visible hydrogen emissions come from the first four wavelengths of the Balmer Series.

• Hydrogen alpha: 656 nm - FF0000
• Hydrogen beta: 486 nm - 55DDFF
• Hydrogen gamma: 434 nm - 0000FF
• Hydrogen delta: 410 nm - 5F5FD3

Creating the color of glowing hydrogen is now a matter of finding the relative intensities of each wavelength.

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/hydspec.html

• 300p H Alpha
• 080p H Beta
• 030p H Gamma
• 015p H Delta
• 425p total

The relative colors for mixing are then:

• 180r 0g 0b
• 16r 42g 48b
• 0r 0g 18b
• 3r 3g 7b

This yields 199r 45g 73b. Maximizing the intensity gives 255r 58g 94b or FF3A5E.

http://spacemandan.net/astronomy/Deep_Space/Eagle_Nebula/In_True_Color-med.jpg

Comparing this color to the picture above, the color seems reasonably accurate.