You might think that with the specification of the triangular model for an objects surface we have achieved our goal. However, as we have defined it, this model only describes the geometry (the shape of the object) like any good model we need to paint the surface to make it look like the real thing.
It turns out that this is not too difficult. You just have to give a few extra numbers for each face to tell the computer the color, or the reflectivity or whatever you think is important.