Add additional colors to the Block Editor color palette.

my_custom_editor_color_palette_extended.php

<?php
/**
 * Add additional colors to the Block Editor color palette.
 * Update and insert additional colors into the $new_colors array.
 *
 */
function my_custom_editor_color_palette_extended() {
	$editor_settings = get_block_editor_settings( array(), 'core/edit-post' );
	$new_colors = array(
		array(
			'name' => __( 'New Color 1', 'textdomain' ),
			'slug' => 'my-new-color-1',
			'color' => '#BF665E',
		),
		array(
			'name' => __( 'New Color 2', 'textdomain' ),
			'slug' => 'my-new-color-2',
			'color' => '#253659',
		)
	);

	add_theme_support( 'editor-color-palette', array_merge( $editor_settings['colors'], $new_colors ) );
}
add_action( 'after_setup_theme', 'my_custom_editor_color_palette_extended', 11 );