@adobe/leonardo-contrast-colors

This package contains all the functions for generating colors by target contrast ratio.

Using Leonardo Contrast Colors

Install the package:

npm i @adobe/leonardo-contrast-colors

Import the package:

CJS (Node 12.x)

const {Theme, Color, BackgroundColor} = require('@adobe/leonardo-contrast-colors');

ESM (Node 13.x)

import {Theme, Color, BackgroundColor} from '@adobe/leonardo-contrast-colors';

Create and pass colors and a background color to a new Theme (see additional options below):

let gray = new BackgroundColor({
  name: 'gray',
  colorKeys: ['#cacaca'],
  ratios: [2, 3, 4.5, 8]
});

let blue = new Color({
  name: 'blue',
  colorKeys: ['#5CDBFF', '#0000FF'],
  ratios: [3, 4.5]
});

let red = new Color({
  name: 'red',
  colorKeys: ['#FF9A81', '#FF0000'],
  ratios: [3, 4.5]
});

let theme = new Theme({
  colors: [gray, blue, red],
  backgroundColor: gray,
  lightness: 97
});

// returns theme colors as JSON
let colors = theme.contrastColors;

API Reference

Theme

Class function used to generate adaptive contrast-based colors. Parameters are destructured and need to be explicitly called.

Setters

Theme.addColor = color

Add a Color to an existing theme

const red = new Color({...})

theme.addColor = red;

Theme.removeColor = color

Remove a Color from an existing theme. Accepts an object with the Color's name and value, or by passing the Color class itself.

// Remove via color name
theme.removeColor = {name: 'Red'};

// Remove via Color class
const red = new Color({...})
theme.removeColor = red;

Theme.updateColor = {name, property}

Update a Color via its setters from the theme. Accepts an object with the name of the color you wish to modify, followed by the property and the new value you wish to modify.

const red = new Color({...})
// Change the colors ratios
theme.updateColor = {name: 'red', ratios: [3, 4.5, 7]};

// Change the colors colorKeys
theme.updateColor = {name: 'red', colorKeys: ['#ff0000']};

// Change the color's name
theme.updateColor = {name: 'red', name: 'Crimson'};

Supported output formats:

Available output formats conform to the W3C CSS Color Module Level 4 spec for the supported options, as listed below:

Color

Class function used to define colors for a theme. Parameters are destructured and need to be explicitly called.

Setters

Supported interpolation colorspaces:

Below are the available options for interpolation in Leonardo:

• LCH
• LAB
• CAM02
• HSL
• HSLuv
• HSV
• RGB

Ratios as an array

When passing a flat array of target ratios, the output colors in your Theme will be generated by concatenating the color name (eg "Blue") with numeric increments. Colors with a positive contrast ratio with the base (ie, 2:1) will be named in increments of 100. For example, gray100, gray200.

Colors with a negative contrast ratio with the base (ie -2:1) will be named in increments less than 100 and based on the number of negative values declared. For example, if there are 3 negative values [-1.4, -1.3, -1.2, 1, 2, 3], the name for those values will be incremented by 100/4 (length plus one to avoid a 0 value), such as gray25, gray50, and gray75.

For example:

new Color({
  name: 'blue',
  colorKeys: ['#5CDBFF', '#0000FF'],
  colorSpace: 'LCH',
  ratios: [3, 4.5]
});

// Returns:
[
  {
    name: 'blue',
    values: [
      {name: 'blue100', contrast: 3, value: '#8d63ff'},
      {name: 'blue200', contrast: 4.5, value: '#623aff'}
    ]
  }
];

Ratios as an object

When defining ratios as an object with key-value pairs, you define what name will be output in your Leonardo theme.

new Color({
  name: 'blue',
  colorKeys: ['#5CDBFF', '#0000FF'],
  colorSpace: 'LCH',
  ratios: {
    'blue--largeText': 3,
    'blue--normalText': 4.5
  }
});

// Returns:
[
  {
    name: 'blue',
    values: [
      {name: 'blue--largeText', contrast: 3, value: '#8d63ff'},
      {name: 'blue--normalText', contrast: 4.5, value: '#623aff'}
    ]
  }
];

Output examples

There are two types of output you can get from the Theme class:

Theme.contrastColors

Each color is an object named by user-defined value (eg name: 'gray'). "Values" array consists of all generated color values for the color, with properties name, contrast, and value:

[
  {background: '#e0e0e0'},
  {
    name: 'gray',
    values: [
      {name: 'gray100', contrast: 1, value: '#e0e0e0'},
      {name: 'gray200', contrast: 2, value: '#a0a0a0'},
      {name: 'gray300', contrast: 3, value: '#808080'},
      {name: 'gray400', contrast: 4.5, value: '#646464'}
    ]
  },
  {
    name: 'blue',
    values: [
      {name: 'blue100', contrast: 2, value: '#b18cff'},
      {name: 'blue200', contrast: 3, value: '#8d63ff'},
      {name: 'blue300', contrast: 4.5, value: '#623aff'},
      {name: 'blue400', contrast: 8, value: '#1c0ad1'}
    ]
  }
];

Theme.contrastColorPairs

Simplified format as an object of key-value pairs. Property is equal to the generated or user-defined name for each generated value.

{
  "gray100": "#e0e0e0";
  "gray200": "#a0a0a0";
  "gray300": "#808080";
  "gray400": "#646464";
  "blue100": "#b18cff";
  "blue200": "#8d63ff";
  "blue300": "#623aff";
  "blue400": "#1c0ad1";
}

Theme.contrastColorValues

Returns all color values in a flat array.

['#e0e0e0', '#a0a0a0', '#808080', '#646464', '#b18cff', '#8d63ff', '#623aff', '#1c0ad1'];

Leonardo with CSS variables

Here are a few examples of how you can utilize Leonardo to dynamically create or modify CSS variables for your application.

Vanilla JS

let varPrefix = '--';

// Iterate each color object
for (let i = 0; i < myTheme.length; i++) {
  // Iterate each value object within each color object
  for (let j = 0; j < myTheme[i].values.length; j++) {
    // output "name" of color and prefix
    let key = myTheme[i].values[j].name;
    let prop = varPrefix.concat(key);
    // output value of color
    let value = myTheme[i].values[j].value;
    // create CSS property with name and value
    document.documentElement.style.setProperty(prop, value);
  }
}

React

Create a new Theme component Theme.js with your parameters:

import * as Leo from '@adobe/leonardo-contrast-colors';

const Theme = () => {
  let gray = new Leo.BackgroundColor({
    name: 'gray',
    colorKeys: ['#cacaca'],
    ratios: [2, 3, 4.5, 8]
  });

  let blue = new Leo.Color({
    name: 'blue',
    colorKeys: ['#5CDBFF', '#0000FF'],
    ratios: [3, 4.5]
  });

  let red = new Leo.Color({
    name: 'red',
    colorKeys: ['#FF9A81', '#FF0000'],
    ratios: [3, 4.5]
  });

  const adaptiveTheme = new Leo.Theme({
    colors: [gray, blue, red],
    backgroundColor: gray,
    lightness: 97,
    contrast: 1
  });

  return adaptiveTheme;
};

export default Theme;

Then import your Theme component at the top level of your application, and pass the Theme as a property of your app:

// index.js
import Theme from './components/Theme';

ReactDOM.render(
  <React.StrictMode>
    <App adaptiveTheme={Theme()} />
  </React.StrictMode>,
  document.getElementById('root')
);

In your App.js file, import useTheme from css-vars-hook and provide the following within your App function in order to format Leonardo's output in the structure required for css-vars-hook.

// App.js
import {useTheme} from 'css-vars-hook';

function App(props) {
  const [lightness, setLightness] = useState(100);
  const [contrast, setContrast] = useState(1);

  const _createThemeObject = () => {
    let themeObj = {};
    props.adaptiveTheme.contrastColors.forEach((color) => {
      if (color.name) {
        let values = color.values;
        values.forEach((instance) => {
          let name = instance.name;
          let val = instance.value;
          themeObj[name] = val;
        });
      } else {
        // must be the background
        let name = 'background';
        let val = color.background;
        themeObj[name] = val;
      }
    });
    return themeObj;
  };

  const theme = useState(_createThemeObject());

  const {setRef, setVariable} = useTheme(theme);

  return <div className="App" ref={setRef}></div>;
}

To make your application adaptive, include a function for updating your theme before your return function:

function _updateColorVariables() {
  let themeInstance = _createThemeObject();

  for (const [key, value] of Object.entries(themeInstance)) {
    setVariable(key, value);
  }
}
// call function to set initial values
_updateColorVariables();

Finally, reference this function and set the theme parameters when your users interact with slider components (do the same for Contrast):

<label htmlFor="lightness">
  Lightness
</label>
<input
  value={lightness}
  id="lightness"
  type="range"
  min={ sliderMin }
  max={ sliderMax }
  step="1"
  onChange={e => {
    setLightness(e.target.value)
    props.adaptiveTheme.lightness = e.target.value
    _updateColorVariables()
  }}
/>
<label htmlFor="contrast">
  Contrast
</label>
<input
  value={contrast}
  id="contrast"
  type="range"
  min="0.25"
  max="3"
  step="0.025"
  onChange={e => {
    setContrast(e.target.value)
    props.adaptiveTheme.contrast = e.target.value
    _updateColorVariables()
  }}
/>

Dark mode support in React

Include the following in your App.js file to listen for dark mode. This will pass a different lightness value (of your choice) to Leonardo. It's recommended to restrict the lightness range based on mode in order to avoid inaccessible ranges and to provide a better overall experience

const mq = window.matchMedia('(prefers-color-scheme: dark)');
// Update lightness and slider min/max to be conditional:
const [lightness, setLightness] = useState(mq.matches ? 8 : 100);
const [sliderMin, setSliderMin] = useState(mq.matches ? 0 : 80);
const [sliderMax, setSliderMax] = useState(mq.matches ? 30 : 100);

// Listener to update when user device mode changes:
mq.addEventListener('change', function (evt) {
  props.adaptiveTheme.lightness = mq.matches ? 11 : 100;
  setLightness(mq.matches ? 11 : 100);
  setSliderMin(mq.matches ? 0 : 80);
  setSliderMax(mq.matches ? 30 : 100);
});

Why are not all contrast ratios available?

You may notice the tool takes an input (target ratio) but most often outputs a contrast ratio slightly higher. This has to do with the available colors in the RGB color space, and the math associated with calculating these ratios.

For example let's look at blue and white. Blue: rgb(0, 0, 255) White: rgb(255, 255, 255) Contrast ratio: 8.59:1

If we change any one value in the RGB channel for either color, the ratio changes: Blue: rgb(0, 1, 255) White: rgb(255, 255, 255) Contrast ratio: 8.57:1

If 8.58 is input as the target ratio with the starting color of blue, the output will not be exact. This is exaggerated by the various colorspace interpolations.

Since the WCAG requirement is defined as a minimum contrast requirement, it should be fine to generate colors that are a little more accessible than the minimum.

Chroma.js

This project is currently built using Chroma.js with custom extensions to supportCIE CAM02. Additional functionality is added in Leonardo to enhance chroma scales so that they properly order colors by lightness and correct the lightness of the scale based on HSLuv.

Contributing

Contributions are welcomed! Read the Contributing Guide for more information.

Development

You can run tests and watch for changes with:

pnpm dev

or run just tests once with:

pnpm test

Licensing

This project is licensed under the Apache V2 License. See LICENSE for more information.