UPDATE: The "red moon planets" appear to be a series of planet arts that are indeed hard-coded (i.e., not in the .xml files). They appear to be referenced in some of the planet definitions, e.g.,
I assume that the random planet generator is using these hard-coded planet arts rather than the generic arts in the regular .xml files. I'm not sure if this explains why one cannot have 100% planets with traits (i.e., 0% generics), but it is a start.
It would be swell if this were fixed in Crusade so that we can mod ALL of the planets.