金光明經文句記 2.171

2.171

二、「如水」下，喻。真金上色，須水銀和，方能塗（去聲）物。闕此一緣，金無塗用。

Second, from "Like water..." onward: the analogy. For the superior color of pure gold, one must mix it with mercury before it can coat objects. Lacking this one condition, gold has no coating function.