You don't. My solid state professor told me that no one ever came up with an accurate model for this. He did his grad work on LED's.
What the industry did is to have grad students cook up thousands of batches with different dopants and concentrations. They then measured the spectrum of the resulting LED's and stored the data in tables. Somewhere there must be a book filled with tables for LED colors.
Edit:
You can get close to the color you want by selecting a dopant which creates an ion center at a specific energy level but there is no formula for getting an exact color.
My information may be out of date though. I took solid state physics about 10 years ago.