Sanitizes a hex color.

Returns either ”, a 3 or 6 digit hex color (with #), or nothing.
For sanitizing values without a #, see sanitize_hex_color_no_hash().

function sanitize_hex_color( $color ) {
	if ( '' === $color ) {
		return '';
	}

	// 3 or 6 hex digits, or the empty string.
	if ( preg_match( '|^#([A-Fa-f0-9]{3}){1,2}$|', $color ) ) {
		return $color;
	}
}