One colour carries heat across every Ordova surface, and it is spent carefully. On the schematics amber rides only the live path; in this room it marks only how recently a record was filed. Warm nodes cool as they descend the spine, and the dark ground is cold storage, the archive at night.

The rule earns its keep by what it forbids. Nothing decorative may be amber, so when amber appears it is information, not ornament. A visitor who learns the rule once can read every page after it.