Rose gold because you could only get it for 2 weeks back about 6 months ago. The queens event was also a bit pointless after they removed the ability to get shards from the gear so not many people got to the required rank. Revanent and the other dead orbit shaders are rare. Same for the new monarchy shaders, all due to low drop rates and most people don't bother with factions. That being said most people do new monarchy because the weapons are better for pvp. Dead orbit is only really for the shaders. I have seen only one person in game using the future war cult shader as not many people work on FWC. So I would say their shaders are the rarest of the factions. (Cue 'oh I have X shader it's not rare' posts.)