The diffuse color is the color that an object reflects when illuminated by "good lighting," that is, by direct daylight or artificial light that makes the object easy to see.

Vase has a mapped diffuse color.

When we describe an object's color in conversation, we usually mean its diffuse color.

The choice of an ambient color depends on the kind of lighting: for moderate indoor lighting it can be a darker shade of the diffuse color, but for bright indoor lighting and for daylight, it should be the complement of the primary (key) light source. The specular color should be either the same color as the key light source, or a high-value low-saturation version of the diffuse color.