Add WordPress theme support for editor-color-palette, and set colours for the gutenberg colour pallete.

change_gutenberg_colour_pallete.php

<?php
/**
 * change_gutenberg_color_palette.
 *
 * Add theme support for editor-color-palette,
 * and set colours for the gutenberg colour pallete.
 * 
 * @see https://since1979.dev/wp-snippet-002-changing-the-gutenberg-color-palette/
 * 
 * @uses add_theme_support() https://developer.wordpress.org/reference/functions/add_theme_support/
 * @uses __() https://developer.wordpress.org/reference/functions/__/
 * @uses array() https://www.php.net/manual/en/function.array.php
 * 
 * @return void
 */
function change_gutenberg_color_palette() {
 
    add_theme_support( 'editor-color-palette', array(
        array(
            'name' => __('Blackish', 'your-textdomain'),
            'slug' => 'blackish',
            'color' => '#323232',
        ),
        array(
            'name' => __('Whiteish', 'your-textdomain'),
            'slug' => 'white',
            'color' => '#eeeeee',
        ),
        array(
            'name' => __('White', 'your-textdomain'),
            'slug' => 'white',
            'color' => '#ffffff',
        ),
        array(
            'name' => __('Dark blue', 'your-textdomain'),
            'slug' => 'dark-blue',
            'color' => '#1d2735',
        ),
        array(
            'name' => __('Blue', 'your-textdomain'),
            'slug' => 'blue',
            'color' => '#00659b',
        ),
        array(
            'name' => __('Light blue', 'your-textdomain'),
            'slug' => 'light-blue',
            'color' => '#4999ca',
        ),
    ));
 
}
 
/**
 * Hook: after_setup_theme.
 *
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 * @uses after_setup_theme https://developer.wordpress.org/reference/hooks/after_setup_theme/
 */
add_action( 'after_setup_theme' , 'change_gutenberg_color_palette' );