color-classes.php

<?php 
/**
 * Add custom colors to Gutenberg.
 */
function tabor_gutenberg_colors() {

	// Retrieve the accent color fro the Customizer.
	$accent = get_theme_mod( 'accent_color', '#fff000' );

	// Build styles.
	$css  = '';
	$css .= '.has-accent-color { color: ' . esc_attr( $accent ) . ' !important; }';
	$css .= '.has-accent-background-color { background-color: ' . esc_attr( $accent ) . '; }';

	return wp_strip_all_tags( $css );
}

color-customizer-control.php

<?php
/**
 * Customizer.
 *
 * @param WP_Customize_Manager $wp_customize the Customizer object.
 */
function tabor_customize_register( $wp_customize ) {
	$wp_customize->add_setting(
		'accent_color', array(
			'default'           => '#fff000',
			'sanitize_callback' => 'sanitize_hex_color',
			'transport'         => 'postMessage',
		)
	);

	$wp_customize->add_control(
		new WP_Customize_Color_Control(
			$wp_customize, 'accent_color', array(
				'label'       => esc_html__( 'Accent Color', '@@textdomain' ),
				'description' => esc_html__( 'Add a color to use within the Gutenberg editor color palette.', '@@textdomain' ),
				'section'     => 'colors',
			)
		)
	);
}
add_action( 'customize_register', 'tabor_customize_register', 11 );

enqueue-block-editor-assets.php

<?php
/**
 * Enqueue theme styles within Gutenberg.
 */
function tabor_gutenberg_styles() {

	// Load the theme styles within Gutenberg.
	wp_enqueue_style( 'tabor-gutenberg', get_theme_file_uri( '/assets/css/gutenberg.css' ), false, '@@pkg.version', 'all' );

	// Add custom colors to Gutenberg.
	wp_add_inline_style( 'tabor-gutenberg', tabor_gutenberg_colors() );
}
add_action( 'enqueue_block_editor_assets', 'tabor_gutenberg_styles' );

enqueue-styles.php

<?php
/**
 * Enqueue theme styles.
 */
function tabor_styles() {

	// Load theme styles.
	wp_enqueue_style( 'tabor-style', get_theme_file_uri( '/style.css' ), false, '@@pkg.version', 'all' );

	// Add custom colors to the front end.
	wp_add_inline_style( 'tabor-style', tabor_gutenberg_colors() );
}
add_action( 'wp_enqueue_scripts', 'tabor_styles' );

gutenberg-colors.php

<?php
/**
 * Advanced Gutenberg block features that require opt-in support in the theme.
 */
function tabor_gutenberg_color_palette() {

	/**
	 * Custom colors for use in the editor.
	 *
	 * @link https://wordpress.org/gutenberg/handbook/reference/theme-support/
	 */
	add_theme_support(
		'editor-color-palette',
		array(
			'name'  => esc_html__( 'Gutenberg Accent', '@@textdomain' ),
			'slug'  => 'gutenberg-accent',
			'color' => esc_html( get_theme_mod( 'accent_color', '#fff000' ) ),
		)
	);
}
add_action( 'after_setup_theme', 'tabor_gutenberg_color_palette' );