Colour correction vs colour grading: what is the difference?

Is color correction the same as color grading?

No, though people use the words loosely and the same tools do both. The difference is the question each one answers.

• Colour correction asks: is it right? It fixes what the camera got wrong or what changed between shots: too dark, too bright, too blue under daylight, too orange under a lamp, a green tint from a fluorescent tube, a cutaway that does not match the talking head.
• Colour grading asks: how should it feel? Warm and nostalgic, cool and clinical, faded like old film, rich like an advert. There is no right answer, only one that suits the piece.

Correction has a target you can measure. Grading has a target you choose.

What does colour correction fix?

Four things, in this order:

1. Exposure. Nothing important crushed to black or clipped to white. The waveform shows it plainly: a pile flat against the top is lost highlight detail.
2. White balance. Something that should be white or grey comes out white or grey, with red, green and blue level on the RGB parade.
3. Contrast. Enough separation between dark and light to look natural, without crushing shadows.
4. Matching. Every shot in a scene sits with the others, so a cut does not jump in colour.

Sharpy treats this as measurement, not taste. Ask it to read a clip and it reports the brightness in stops over middle grey, the colour balance and the darkest, middle and brightest thirds of the picture. Click something that should be neutral, a white wall or a grey shirt, and it sets the white balance from it. It reads the light before any grade too, from the key-to-fill ratio to the lighting pattern on a face, and says what post can and cannot fix. And it judges room, face and background against each other: a colour cast thrown by a wall is fixed at the balance stage, and the face follows.

What does colour grading add?

Character. Grading is where a talking head in a white room becomes warm and intimate, or a product shot cool and precise. A grade can be a look, a LUT, or hand-built curves and wheels.

The risk is skin. A warm look pushes skin orange; a cool one makes it grey; a strong film look can turn a face a colour nobody's face is. Sharpy's answer is to grade the room, not the person: moods such as cinematic, warm and cool are laid around the person by default, found in every frame on your Mac with no green screen. On a test recording, the moods laid over the whole picture moved the speaker's skin by 1.5 to 5.5 on the ΔE2000 scale, where about 1 is the smallest difference most people can see. Laid around him, they moved it by less than 0.1.

Colour grading in Sharpy is the lesson on looks, LUTs and the scopes, and what a LUT is explains camera LUTs, which correct, and creative ones, which grade.

What comes first, color correction or grading?

Correction, then matching, then grading. A look is built for a balanced picture. Lay a warm look over a shot that is already orange and you get orange skin, not warmth; lay a contrasty look over a shot that is already crushed and the shadows go to mud.

Sharpy grades in the fixed order colourists use, so a creative look never goes on before the picture is balanced:

1. Noise reduction.
2. Input transform, such as turning a camera's log footage into normal colour.
3. Exposure and white balance.
4. Contrast.
5. Balance and shot matching.
6. Creative look.
7. Skin and secondaries.
8. Grain and output.

Matching sits in the middle because it needs corrected shots and must come before a look. Ask Sharpy which clip does not sit with the rest and it names every clip 1 ΔE2000 or more away, worst first, with the change that brings it in. Ask it to match one shot to another and the reply says how much of each gap closed: on a test recording, a landscape B-roll shot of a keyboard came from 34.2 to 2.1 ΔE2000 away from the talking head in its middle third. How to colour match video clips walks through it.

How do you keep skin tones right through both?

Watch the vectorscope. Skin of every complexion falls near one line on it, the skin line, so a face drifting towards green or magenta shows before your eyes notice. Then avoid two common errors:

• One target for every face. Skin varies in brightness and saturation by complexion. Driving every face to the same numbers is the mistake, not the fix. Sharpy measures each face against the right range for its complexion and leaves it alone when it is already right.
• Grading the face along with the room. If the mood is meant for the scene, keep it off the person.

Every colour change in Sharpy also reports how far it moved the face. At 1 ΔE2000 or more it warns that anyone will see it, which is right when you are removing a cast and wrong when you only meant to change the room.