Stepping back, even if metallic dragons are willing to fight to the death for Muria, Muria can't just send them to their deaths; instead, he needs to keep them safe.
This is because each metallic dragon represents a metallic dragon clan, with their forces deeply intertwined. Thus, Muria has to be cautious using metallic dragons. Any carelessness would lead to questions and disputes. In short, if he were to use metallic dragons, Muria foresees future difficulties.
However, the Five Colored Dragons don't have these concerns. Most of them have no parents, and if they do, they don't acknowledge each other. After domestication and education by Muria, and satisfying their wants and needs, sending them to fight fiercely on the battlefield doesn't necessitate too much concern and doesn't risk an ancient dragon holding him accountable.
