Ambient is simply an overall colour that adds to the shading of a cloud. It's a convenient way of adding brightness to the overall cloud layer in it's simplest application, but can be used to make them a solid colour if you switch off any lights (including the sun) and whack up the Ambient colour to say pure white. It's a great way to make RGB ID channel masks for VFX needs (pure red, green, or blue for instance), by making one layer pure red, another pure green, etc. The RGB channels of the render can then be used to split each cloud layer into separate tweakable elements. The same can be done with the Atmosphere, and even Surface Layers.
