Colour and gradients

How colour is stored, why gradients band, and what to do about it.

Colour is mostly invisible as a system until something looks wrong — a gradient bands, a fade goes muddy in the middle, an export looks flatter than the canvas. All three have the same small set of causes.

Gradients#

A gradient is a list of stops, each with a colour and a position. Between two stops the colour is interpolated. Where you put the stops matters more than how many you use — three well-placed stops beat eight arbitrary ones.

• Linear ramps along an angle.
• Radial spreads from a point.
• Conic wraps around a point, which is what a colour wheel or a sweeping highlight needs.

The muddy-middle problem#

Interpolating between two saturated complementary colours passes through grey. Blue to orange goes through a dead zone in the middle that neither end predicts.

The fix is a third stop. Put a saturated colour at the midpoint — for blue to orange, something like magenta or a warm purple — and the ramp travels around the desaturated centre rather than through it.

Banding#

A large area with a subtle gradient shows visible steps, because there are only 256 values per channel and a slow ramp across 1000 px has to reuse each one about four times.

Two fixes, in order of preference:

1. Add a little Noise / Grain. Three to five percent breaks up the step edges and is invisible as grain. This is what film did for free.
2. Make the gradient travel further in colour, so it uses more of the available range over the same distance.

Where colour changes hands#

Effects run on the finished picture, so a colour adjustment applies to everything below it in the stack — including other effects. Adding Levels above a glow grades the glow too. If that is not what you want, the adjustment belongs on a parent instead.