The IA Color Contrast control is a custom WP_Customize_Control for use in the WordPress Customizer.

It is intended for use by WordPress theme developers who would like to add a contrast check to their theme color options in the customizer. Because it is intended to be used as a part of a custom theme, this repository is not a full plugin. Instead, theme developers are encouraged to drop the files into a theme project and alter the paths and text domain. This control should be compliant with the Theme Review guidelines for wordpress.org theme submission.

The code is intended to be added to the theme's includes directory (the directory is named /inc in underscores, the starter theme from WordPress), then require_once in the theme's customize.php file (again, in underscores, /inc/customize.php).

If you add the files in your theme at another location, you will need to update the paths to the JavaScript and CSS files. These are located in the class-ia-color-contrast-customize-control.php file, in the enqueue method.

If you would prefer to use the text domain of your theme, you can replace the domain ia-color-contrast with your own. There should be five instances in the class-ia-color-contrast-customize-control.php file (in the call to wp_set_script_translations and in the HTML portion of the content_template method) and four instances in the ia-color-contrast-customize-control.js file (in the update method).

If at all possible, please leave the PHP, JavaScript and CSS file comment headers in place.

Before you get started, you should be familiar with the basics of the using the WordPres Customizer API to create custom colors within your theme.

This control is intended to be paired with two WP_Customize_Color_Controls and their settings. Add the two settings IDs to in the options parameter of the IA_Color_Contrast_Customize_Control constructor.

The IA_Color_Contrast_Customize_Control is designed to 'stick' to the previously added WP_Customize_Color_Control, so should be added to the customizer immediately after, or given a priority after, the corresponding WP_Customize_Color_Control.

$wp_customize->add_control( 
   new WP_Customize_Color_Control( 
       $wp_customize, 
       'link_color',
        array(
           'label'    => 'Link Color',
           'section'  => 'theme_colors',
           'settings' => 'link_color',
        )
   )
);
$wp_customize->add_control( 
   new IA_Color_Contrast_Customize_Control( 
       $wp_customize, 'body_text_color_contrast', 
       array(
           'section'  => 'theme_colors',
           'settings' => array(
               'body_background_color',
               'body_text_color'
           )
       ) 
   )
);

Here's a fully functional (if simplistic) customize.php example:

require_once get_theme_file_path( '/inc/ia-cc/class-ia-color-contrast-customize-control.php' );

function my_theme_customize_register( $wp_customize ) {
    $wp_customize->add_section( 'my_theme_colors', array(
		'title'    => 'Theme Colors',
		'priority' => 30,
	) );
    $wp_customize->add_setting( 'body_background_color', array(
		'default'   => '#fff',
		'transport' => 'refresh',
	) );
	$wp_customize->add_setting( 'body_text_color', array(
		'default'   => '#555',
		'transport' => 'refresh',
	) );
    $wp_customize->add_control( new WP_Customize_Color_Control( $wp_customize, 'body_background_color', array(
		'label'    => 'Background',
		'section'  => 'my_theme_colors',
		'settings' => 'body_background_color',
	) ) );

	$wp_customize->add_control( new WP_Customize_Color_Control( $wp_customize, 'body_text_color', array(
		'label'    => 'Text',
		'section'  => 'my_theme_colors',
		'settings' => 'body_text_color',
	) ) );
	$wp_customize->add_control( new IA_Color_Contrast_Customize_Control( $wp_customize, 'body_color_contrast', array(
		'section'  => 'my_theme_colors',
		'settings' => array(
			'body_background_color',
			'body_text_color'
		)
	) ) );
}
add_action( 'customize_register', 'my_theme_customize_register' );

function my_theme_customizer_css() {
	?>
	<style id="customizer-styles" type="text/css">
		body {
			background-color : <?php echo get_theme_mod('body_background_color', '#fff'); ?>;
			color            : <?php echo get_theme_mod('body_text_color', '#555'); ?>;
		}
	</style>
	<?php
}

add_action( 'wp_head', 'my_theme_customizer_css' );

Much of the code used in the JavaScript files originated with the article Building your own color contrast checker by Alvaro Montoro.

As indicated in the article above, the rgbToHex method originated with Tim Down on StackOverflow.