Emission & Ambient

What is the difference between Emission and Ambient colors?

In Opengl emission (glMaterial*(…,GL_EMISSION,…)) the light emitted “from the geometry itself” - there is no need to have other light sources defined in the scene.
The ambient material settings define how much of the ambient light in the scene is REFLECTED from the geometry/faces. Without a lightsource with an ambient part or a globally defined ambient light (glLightModelfv(GL_LIGHT_MODEL_AMBIENT,…)) the ambient material doesn’t reflect anything and will be dark. In the light equation for the material you have sth like

Light = emissive_material + ambient_material*ambient_lightsource_intensity

Details in the redbook: Redbook: Lighting