I'm rendering a cube with a single light-source with the standard white/grey diffuse-shader and therefore getting a picture with values-only and then multiplying a color on the image, the color on the different surfaces just changes its brightness.

But when I'm giving the diffuse material the color I have multiplied on the first render, in the new image the color just doesn't change its brightness, but also its saturation.

Why? Is there just a simple difference I didn't recognize? 1st picture: Values multiplied with color. 2nd picture: Used the color as material for the diffuse-shader.

EDIT: I found out that monochromatic colors dont change their saturarion.