I'm designing a few planets and the life that could inhabit them, and was wondering if an uncommon Earth phenomenon like iridescent clouds might be more common on a planet under different conditions. Are there any atmospheric properties that would make iridescent clouds a more common occurrence? I think it might be cool to include this as a feature on one of my planets.
The key to having strong iridescence is the uniformity in the distribution of the droplet or ice crystal size and that the they are relatively small. A strong light helps make the phenomenon more visible and the cloud can not be optically thick or you won't get a strong effect. By relatively small, most cloud droplets are between 1 and 10 um but iridescent clouds the droplets are probably less than 1 um or around 1 um. If the particle size is changing you may see a shift in color.
So for your world a bright sun and perhaps strong upper atmospheric winds to shear the clouds so they don't get too thick. The effects would be most strong when looking in directions near the sun.
To keep the droplets more uniform, I think not having as many vertical convection currents within the clouds might help to keep the particles small. Of course you have several different cloud types and you still may want to have thunderstorms.
Since the droplets in clouds often nucleate around a small particle which needs to be less than 1 um, it might be that you have some special way to introduce a special uniform particulate size. Maybe some kind of air-born microbe, or dust particle from some dessert where for some reason the dust is especially fine, or a volcano emitting something very fine.