They aren't exactly rare as they can be spawned. This would be more appropriate for the tamers forum, but I'll give you my opinions anyway.
1) neon green hiryu - 2-3m
2) neon pink hiryu - 3-5m
I don't know why there seems to be more greens than pinks; they both share the same spawn chance (.05?) or something like that. If they have good stats/resists, you could easily get more since tamers are always looking for highend coloured pets.