
Ok, so this one isn't new (original source - Rich Tabor), but I've added it here again, partly so that I can find it, but also in case someone else finds it useful. I find that it's really helpful to limit the colour pallete that's available in Gutenberg - partly so that the brand colours are readily available, but also to prevent the over-enthusiastic from trying to be too creative ;)
On this site, I have the following colours (you can add more if you want). To achieve this, simply add the following to your theme functions.php file.

/**
* Add support for custom color palettes in Gutenberg.
*/
function tabor_gutenberg_color_palette() {
add_theme_support(
'editor-color-palette', array(
array(
'name' => esc_html__( 'Green', '@@textdomain' ),
'slug' => 'green',
'color' => '#7bba66',
),
array(
'name' => esc_html__( 'White', '@@textdomain' ),
'slug' => 'white',
'color' => '#FFFFFF',
),
array(
'name' => esc_html__( 'Black', '@@textdomain' ),
'slug' => 'black',
'color' => '#000000',
),
array(
'name' => esc_html__( 'Light Grey', '@@textdomain' ),
'slug' => 'light_grey',
'color' => '#e6e7e8',
),
)
);
}
add_action( 'after_setup_theme', 'tabor_gutenberg_color_palette' );
// disable custom colours
add_theme_support( 'disable-custom-colors' );
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