CMYK

CMYK (cyan, magenta, yellow, key/black) is the subtractive process model used by four-color print. Each channel is the percentage of one ink laid on a white substrate. K is the black plate, separated out so that dark colors do not require saturating all three chromatic inks.

Coordinates

Channel	Range	Meaning
C	[0, 1] or [0, 100] percent	cyan ink coverage
M	[0, 1] or [0, 100] percent	magenta ink coverage
Y	[0, 1] or [0, 100] percent	yellow ink coverage
K	[0, 1] or [0, 100] percent	black ink coverage

CMYK is device-dependent. A real conversion needs an ICC profile for the target press, paper, and ink set. The browser-side approximation iridis uses is the standard naive transform with no profile applied, accurate enough for a UI control, not accurate enough to send to a printer.

The K channel

Naive subtractive RGB → CMY would set C = 1 − R, M = 1 − G, Y = 1 − B. That works mathematically but is wasteful in practice: stacking three inks to render dark grey costs ink, dries slowly, and shifts hue. The K channel exists as the common minimum that gets pulled out of all three:

K = 1 − max(R, G, B)
C = (1 − R − K) / (1 − K)
M = (1 − G − K) / (1 − K)
Y = (1 − B − K) / (1 − K)

When K = 1 (pure black), the chromatic channels are forced to zero to avoid division by zero. The reverse:

R = (1 − C) · (1 − K)
G = (1 − M) · (1 − K)
B = (1 − Y) · (1 − K)

Math primitives

iridis does not export CMYK math primitives. CMYK exists only in the picker as a numeric I/O affordance, designers occasionally want to see what a screen color looks like in CMYK terms even when the output is digital.

Where it appears

• IridisPicker.vue (docs/.vitepress/theme/components/IridisPicker.vue) provides a CMYK tab. Edits convert through CMYK → RGB → hex and emit a fresh hex on every change.
• The pipeline never receives CMYK input. There is no intake:cmyk task and no CMYK channel on ColorRecord.