Why are HUDs usually green instead of red?
A few thoughts (a bit of personal research):

Eyesight


The eye is most sensitive to green color, as can be seen in this diagram:





  At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength.



(source: wikimedia.org)  

Dotted R line is (night-vision) human rhodopsin 'rods'.

As I understand, any powerful light depletes rhodopsin down requiring that they regenerate, which takes time, just that depletion takes longer with red wavelengths. I think this does not imply that green color 'automatically' screw up night vision if the display intensity is lowered enough.

Technical Limitations


The light has to be collimated to not interfere with depth perception, which complicates having multiple colours. At least in old CRT displays, this would have left you with the choice between blue, red or green. Green and Red is perhaps the color which stands out the most, but this diagram would suggest the red occurs at different wavelengths, perhaps again interfering with collimation:



  Due to the complexity of wide field-of-view reflective HUD optical systems, the optical designer must use all means available to meet display accuracy and parallax error requirements. All certified reflective HUDs today are monochromatic, generally using a narrow-band green emitting phosphor. The addition of a second color to the HUD is a desirable natural progression in HUD technology, however, one of the
  technical challenges associated with adding a second (or third) display color is maintaining the performance standards available in monochrome displays. One method for solving this problem uses a collimator with two independent embedded curvatures, one optimized for green symbology, the other optimized for red symbology, each with a wavelength-selective coating. Avionics Handbook, Chapter 4



Compounding to the previous point, it may be difficult to filter through a single red color through the screen, ending with a rather broad spectrum instead:




Further thoughts:


I think the night vision would be disturbed anyway by the variety of instruments such as maps and radar. I'm also doubting a bit of the importance of night vision in combat situations.
Night vision googles also use monochrome green light, despite having perhaps the greatest benefit of red light should they break and you need to continue without.
Even in a pretty new BMW I drove there was only monochrome green from their own HUD. I read forums accusing systems featuring multiple colours as having inferior display quality.