Some cults use partial Runes the God Learners believed were not ‘true’ Runes but merely expressed limited subsets of Elemental Runes. As such, they described a limitation on a god’s power, rather than a fundamental building block of the cosmos. Despite this, they fulfill particular needs and are commonly accepted.
Light: The Light Rune represents the Fire/Sky Rune shorn of Fire. It is light without Fire.
Heat: The Heat Rune represents the Fire/Sky Rune shorn of Sky and Light. It is fire without light.
Cold: The Cold Rune represents the Darkness Rune shorn of Shadow. It is cold, but not dark.
Shadow: The Shadow Rune represents the Darkness Rune shorn of Cold. It is dark, but not cold.
When an adventurer wants to use a partial Rune, the rating with the full Elemental Rune is used. For example, an adventurer with a Fire/Sky Rune of 90% joins the Yelmalio cult, foreswearing the Fire aspect of the Fire/Sky Rune. They use their rating with the Fire/Sky Rune whenever the Light Rune is called upon.
Would trollkin prefer Shadow to Darkness? Not particularly – Xiola Umbar is deep in the Darkness and she’s the only one who really cares about trollkin. Argan Argar is sometimes associated with Shadow as well, and he doesn’t hate trollkin, just considers them to be tools.
Is the Trade rune, a partial Harmony rune? The Trade Rune is recognised pretty much only by the Issaries cult. It is Movement and Harmony.