Primary Gel Wash

Coloured light on a dark set separates space without any other help. A red wall and a blue wall are read as two rooms even when the camera never shows the join.

It is also the fastest way to make an ordinary room feel wrong. Nothing in the frame is a strange shape, only a strange colour, and a viewer cannot argue with light.

Assign one hue to one surface. Red on this wall, blue on the next, green behind. Mixing two gels on the same surface gives mud and loses the whole effect.

Keep the shadows black. This palette has no mid grey in it: a surface is either washed in one colour or it is dark.

Allow exactly one white source if you need a face to read, and say it is a practical lamp inside the frame. More than one and the coloured light turns decorative.

Use it for horror, for a nightclub, for a dream, for a music video, for any interior that has to stop feeling like a room.

Leave it alone for anything warm or domestic. Coloured light on a dark set cannot be made comfortable.

Amber and Cyan uses two colours that both come from real street lighting, so it stays plausible. This one uses colours no real room has and does not pretend otherwise.

Night Blue and Magenta washes the whole frame in a single night. Here each surface takes its own hue, so the frame is divided rather than unified.

Italian horror of the 1970s used it because a gelled lamp was cheaper than a set, and the result outlived the films. The same trick came back forty years later in music video and in nightclub scenes, for the same reason.

The whole frame comes back tinted one colour. Say the light comes from lamps inside the room and that each surface takes a different hue.

The shadows lift to grey and the picture goes flat. Say the shadows stay black and there is no white light.