How to define custom colour palette for Gutenberg; the new WordPress block editor. More info see https://philkurth.com.au/tips/how-to-customize-the-colour-palette-for-wordpress-gutenberg-block-editor/

define-custom-colour-palette-for-gutenberg.php

<?php

add_action( 'after_setup_theme', function () {

	add_theme_support( 'editor-color-palette', [
		[
			'name' => 'Purple',
			'slug' => 'purple',
			'color' => '#2B265C'
		],
		[
			'name' => 'Mauve',
			'slug' => 'mauve',
			'color' => '#915099'
		],
		[
			'name' => 'Charcoal',
			'slug' => 'charcoal',
			'color' => '#222222'
		],
	] );

} );