Colour and Accessibility

Farah's warehouse table colours each cell green when it meets its target and red when it misses. It looks tidy on Ishaan's screen. Then Kabir, the data engineer, says he cannot tell which cells are which: he is one of the roughly 1 in 12 men with red–green colour blindness. The status he needs is carried by colour alone, and for him that colour is not there.

The rule

Grey by default, colour with a reason, never colour alone.

Colour is the strongest signal on a chart, so spend it carefully:

• Grey by default. Draw most of the chart in greys, then use one accent colour for the thing the viewer must notice, such as the one warehouse below target.
• Colour with a reason. Match the palette to the data:
  • Categorical: clearly different hues for groups with no order, such as product categories. Keep to a handful; beyond about six, people cannot tell them apart or remember which is which.
  • Sequential: light to dark shades of one hue for low to high amounts, such as orders by state.
  • Diverging: two hues that meet at a meaningful middle, such as on-time rate above and below the 95% target.
• Never colour alone. Pair colour with something that survives without it: a text label, a symbol such as ▲ or ▼, a pattern, or position.

Accessibility means everyone can read the chart, including people with colour blindness, low vision, small screens or screen readers:

• Avoid pairing red with green. Blue and orange that also differ in lightness are much safer.
• Keep text contrast at least 4.5 to 1 against its background, the common guideline for normal-sized text, and make chart marks clearly distinct from their background.
• Label lines directly instead of relying on a colour legend.
• Add a one-sentence summary of what the chart shows, which screen readers can read aloud.

The shortcut

Run the greyscale test: imagine the chart printed in black and white. If any meaning disappears, colour was doing that job alone. Add a label, symbol or position until the chart survives.

Drills

Answer fast — trust the rule.

Remember

• Grey by default, colour with a reason, never colour alone.
• Greyscale test: if meaning vanishes in black and white, add labels, symbols or position.
• Trap: red–green pairs and colour-only status cells.