Mark, While the predominant colors are green and gold from particle interaction with atmospheric oxygen, blues occur when nitrogen is excited. Because red is a slower longer wavelength of light it frequently is overlooked in an image. To get reds, one usually needs a longer exposure.
Below is from NASA
The color of the aurora depends on which gas - oxygen or nitrogen - is being excited by the electrons, and on how excited it becomes. Oxygen emits either a greenish-yellow light (the most familiar color of the aurora) or a red light; nitrogen generally gives off a blue light. The blending of these colors can also produce purples, pinks, and white. The oxygen and nitrogen also emit ultraviolet light, which can be detected by special cameras on satellites but not by the human eye. Source: NASA