Why is color alone not enough to convey information?

WCAG 1.4.1 Use of Color (level A) says color must not be the only way to show information, trigger an action, or tell elements apart.

Many users cannot rely on color:

• People with color vision deficiency, which affects roughly 1 in 12 men and 1 in 200 women.
• People with low vision, or using high contrast or grayscale modes.
• Anyone with a monochrome display or bright sunlight on the screen.
• Screen reader users, who get no color at all.

Common fixes:

• Form errors: not just a red border. Add an icon and an error message in text.
• Required fields: write "(required)" instead of only a red asterisk.
• Links in body text: keep an underline, or another cue besides color.
• Charts: add patterns, shapes or direct labels.
• Status: pair a colored dot with a word such as "Online" or "Failed".

<label for="email">Email</label>
<input id="email" aria-invalid="true" aria-describedby="err">
<p id="err"><svg aria-hidden="true">...</svg> Enter a valid email address.</p>

To test, view the page in grayscale or emulate color blindness in DevTools.

Use color to support meaning, never to carry it alone.