I have had this theory cooking in my had for a while, and now while reading the Taldain chapter in Arcanum Unbounded (I have had the UK audiobook version so haven't read this before) and than seeing your theory and the mention of the bone spores it clicked!
From the Taldain systen interlude:
«The Investiture beats down from the sky, and is absorbed by a microflora that grows like a lichen on the surface of the sand, giving it its brilliant white color (when fully Invested) or deep blackness (when that Investiture is depleted).
Giving water to the tiny plant causes a chain reaction of sudden growth, energy, and Realmic transition. Certain people can control this reaction, using the water from their own bodies to forge a brief Cognitive bond. They can draw Investiture (in very small amounts) directly from the Spiritual Realm, and use that to control the sand.
Though the effect is dramatic, the actual power used is quite small. This is a magic more about finesse than raw strength.»
And from Kanton’s description of the same color in chapter one of the excerpt:
«With an explosion of light, the sand changed from bone white to shimmering mother-of-pearl.»
I love the Cosmere and this small hints Branden puts into his works! Would love if this could be addressed in a WoB
