Guttenburg - Add custom colours

guttenburg-colours.css

/** ANCHOR Custom colours for Gutenberg editor */

/** Blue colours **/
.has-blue-background-color {
	background-color: #009aa9;
}
.has-blue-color {
	color: #009aa9;
}

guttenburg-colours.php

<?php
function elab_guttenburg_colours() {
	// Disable Custom Colors
	add_theme_support( 'disable-custom-colors' );
  
	// Editor Color Palette
	add_theme_support( 'editor-color-palette', array(
		array(
			'name'  => __( 'Blue', 'elab' ),
			'slug'  => 'blue',
			'color'	=> '#009aa9',
		)
	) );
}
add_action( 'after_setup_theme', 'elab_guttenburg_colours' );