Green is the color of natural law, of true justice. When you're in a situation that requires you to be in court, or dealing with any judicial matters, or you have a need for justice, then the green Santa Muerte is the aspect to pray with. When you need to "nail" something down, to be done with it, put a nail in the coffin of a matter.
She helps things come into balance.
According to Tomas Prower, she is also helpful when praying for help with autoimmune disorders and other immune depressing diseases (such as HIV/AIDS).