Describing color and contrast in alt text without over-describing

The quarterly deck has a bar chart with six bars. Four are teal and two are red, and a note under the chart explains that red means the rep missed quota. The alt text somebody typed in a hurry reads "Bar chart of Q3 sales by rep." Everything a person would actually repeat back in the meeting — who missed — just fell out of the picture.

One rule settles most color questions: describe color when the color is carrying information, and leave it out when it's only what the thing happens to look like. This isn't a matter of taste. W3C catalogs the first half as a named failure — F13, a text alternative that omits information conveyed by color differences — and its worked example is almost exactly the sales chart above.

When color is doing a job

Color is carrying information any time it stands in for a label. Red bars mean missed quota. Green dots mean rooms available. Shaded regions on a map mean different zones. In every case a legend somewhere translates a hue into a word, and your image description has to do that translation for the reader instead of handing them the hue. That's most of the work in alt text for charts and infographics — you're reading the key, not the picture.

The other case is when the color is the product. A shopper deciding between the sage and the rust version of the same jacket needs that word — it's the entire basis of the decision, and it's why describing product photos nearly always includes color, material, and shape.

One distinction worth keeping straight, because people conflate them constantly. WCAG's Use of Color criterion is about sighted readers: it asks that information carried by color also be available through some other visible means, like a label or a pattern, since plenty of people don't distinguish the hues you picked. The National Eye Institute puts it at about 1 in 12 men, red-green being the most common type. That's a design fix, made in the design file. Alt text is the separate question of whether the meaning survives for someone not looking at the image at all. A color-coded chart usually fails both at once, and fixing one doesn't fix the other.

When color is just what it looks like

Here's the over-describing half, and it's the more common failure by a wide margin. W3C's tutorial on informative images says the text alternative should convey the meaning displayed visually, which "typically isn't a literal description of the image." Shirt colors, wall paint, the shade of the sky, the exact blue in the mug — a sighted reader glances past all of it, so reading it aloud isn't equal access. It's a longer wait for the same information.

The test is the same one you'd apply when deciding whether an image is decorative or informative: ask what the picture is there to do. If a detail isn't part of that job, it isn't earning its place in the description. Every unnecessary adjective pushes the useful part further down — which is the instinct behind the 125-character rule, even on platforms with a much bigger box.

Contrast almost never belongs in the description

Contrast is a different animal. "Pale gray text on a white background" describes a problem without solving it — the person listening still doesn't know what the text said, and nothing about the description makes the graphic easier to read for the people who were squinting at it. Put the words in your alt text, and fix the contrast in the source file where it actually helps.

The exception is when contrast is the subject. A before-and-after screenshot in an accessibility audit, a slide showing two versions of a button — there, the difference in legibility is the whole point, and it goes in the description because it's the content. Same rule as always, just applied to an unusual image.