Ambient light is one that has no position nor direction and, therefore, a facet cannot be "turned away" or "hidden" from the light. In turn, all points on all objects are illuminated equally by an ambient light. Due to this, the computation of the reflected color simply blends the material's ambient color component with the ambient light's color. In particular, the perceived color of a surface due to reflection of ambient light is computed with the Phong Illumination Model, using the formula
I = M*L
where M is the material's ambient color and L is the ambient light's color. The computation is performed for each color component (R, G and B). It is easy to see that the formula does give the desired model and, in particular, all points of equal color will produce the same reflected color. Also, a color component of the ambient light will blend with the corresponding component of the material by producing one of equal or lesser value. For example, if the red in the material is 1.0 and the red in the ambient light is 0.5, then the amount of red we see reflected from the surface is
I = 1.0*0.5 = 0.5
As another example, we will not see a yellow material with a blue ambient light. WHY?