A scumble is a thinly applied paint layer which is lighter in tone than the underlying colour (as opposed to a glaze, which is darker).
A scumble is translucent because of its thinness rather than because of the optical nature of the pigments and medium.
A paint mixture scumbled over a dark underlayer will appear cooler and more blue in hue than the same colour over a lighter ground or underpaint. For the same optical reasons a column of smoke seen in front of, for example, some dark trees, will appear blue, whereas in front of a pale sky it will appear warm grey or white.