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Actually, Bruno had quite a lot of say about this on Discord, but <a rel="nofollow" class="external text" href="https://www.reddit.com/r/fallenlondon/comments/1cotrj5/neathbow_icons/l3gzgmu/">this comment on Reddit</a> has summarized some of the salient points, particularly
- Chthonosophy, SA, and SoTD are not Neathbow colors, though the rest are.
- Irrigo is more associated with Midnighters/the Nadir than APoC, which is associated with apocyan via Principles of Coral
I'm personally a bit surprised by the lack of cosmogone as well! But since it's most closely associated with the parabolan sun, which is artificial, perhaps FBG view it as a similar case as irrigo and the Nadir.