Desaturation

Taking colour out takes energy out. A desaturated frame reads as tired, cold, documentary or remembered, depending on what else is happening inside it.

Leaving a trace behind is what separates it from black and white. The eye still registers the world as coloured, and then notices that the colour has drained away.

Pull the saturation down as a whole and stop short of zero. Somewhere between a quarter and a half of the original is the usual working range, which keeps a hint of warmth in skin while everything else goes close to grey.

Deciding what keeps its colour is the real choice. Holding skin while dropping the rest keeps faces alive in a grey world. Dropping skin as well makes people part of the same drained picture.

Contrast is a separate dial. Desaturation on its own leaves the tonal range where it was, and that is what keeps it apart from a bleach bypass.

Use it on realism, on grief, on winter, and on flashbacks that should sit apart from the present without going fully monochrome.

Skip it when the set or the costume is doing the work. Anything chosen for its colour stops paying for itself once the saturation comes down.

Bleach bypass drains colour and raises contrast at the same time, so the blacks crush and the highlights go metallic.

Plain desaturation leaves the contrast alone. Look at the blacks. Open and grey means desaturation. Crushed to nothing under hard bright highlights means bleach bypass.

Full monochrome comes back, because a model reads desaturated as no colour at all. Say how much is left, such as a quarter of the original, and name where it survives.

The other failure is a flat grey frame with nothing in it. Ask for the contrast to stay normal so the picture keeps its shape after the colour goes.