As I understand it the health of a tier or node (in terms of RAG status) is determined by whether there are health rule violations that affect those tiers or nodes. So a Critical health rule violation against tier X will make tier X go red, and a war...
As far as I can tell you have to add them in the order you want to display them, so you have to know which is the higher numbered metric before you add them and add them in order, which is far from ideal.