This is an extension of yesterdays overview on lighting.
This model is pretty much the defacto-standard kicking around today. It's also pretty similar to what you can get if you use the Direct3D fixed-function lighting. Seems like a good place to start.
There are a few similar variants kicking around, and this following equation is the one I'm going to use:
The subscript 'I' (Intensity) and 'C' (Colour) parts can be rolled into one, making there 3 colours in the equation: Ambient, Diffuse and Specular.
All fitting into this lovely Visio diagram: