Halos are not exclusive to ice crystals, and rainbow colored halos are commonly reported, they usually indicate some issue with eye's focusing. One possibility is a growing cataract:"Seeing rainbows or halos around light indicates a problem with how light is filtering into the eye. Light is made up of different colours but the rays are normally focused on a single point so you can't distinguish them. 'These symptoms indicate that scattering of light is occurring on the cornea or lens,' says Dr Konrad Pesudovs, the foundation chair of optometry and vision science at Flinders University's School of Medicine, Australia... This could be due to opaque spots on the cornea - such as cataracts, where the lens becomes cloudy. This causes light that would normally pass into the eye in a straight line to scatter or bend. This has the effect of a blurry halo or rainbow as the light is split in different directions. 'Cataracts will eventually require surgery,' says Dr Pesudovs".
Although check-up is probably in order do not panic yet. In response to one Simon Kay reporting an issue similar to yours ask-the-optician gives a similar but less ominous explanation:"The eye is not perfect and doesn't always cope well with strong lights on a dark background. Monochromatic lights (lights of one colour) are focused at different distances from the retina and therefore two coloured lights together may not be both in focus at the same time. You may have slightly hazy media, like in the cornea, the aqueous or the vitreous gel... not enough to be abnormal, but enough to cause this problem. You may have quite large pupils at night which expose more of the focusing part of the eye and this creates more aberrations (or faults) in your focusing ability." The haziness may also be amplified by water droplets in the air, as with the fogbows.
