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.