Change a hex value by a positive or negative percentage, whether it is 3 or 6 characters in length. Percentage based on max (255), not self. So 100% will always be "#FFFFFF", -100% will always be "#000000"

hex-tint.php

<?php
/**
 * Tint hex color by a percentage
 *
 * @param string $hex - #FFFFFF or #FFF
 * @param int    $percentage - Positive or negative
 *
 * @return string Returns modified $hex, lighter or darker
 */
function hex_tint( $hex, $percentage=0 ) {
	if ( empty( $hex ) || 0 == $percentage || ! is_int( $percentage ) ) {
		return $hex;
	}

	if ( '#' == substr( $hex, 0, 1 ) ) {
		$temp_hex = trim( $hex, '#' );
		$trimmed = true;
	} else {
		$temp_hex = $hex;
		$trimmed = false;
	}

	if ( 3 == strlen( $temp_hex )  ) {
		$temp_rgb = str_split( $temp_hex, 1 );
		$rgb = array(
			$temp_rgb[0],
			$temp_rgb[0],
			$temp_rgb[1],
			$temp_rgb[1],
			$temp_rgb[2],
			$temp_rgb[2],
		);
		$rgb = str_split( implode( $rgb ), 2 );
	} elseif ( 6 == strlen( $temp_hex ) ) {
		$rgb = str_split( $temp_hex, 2 );
	} else {
		return $hex;
	}

	foreach ( $rgb as $key => $value ) {
		$value = hexdec( $value );
		$value = str_pad( dechex( max( min( ( ( ( $percentage / 100 ) * 255 ) + $value ), 255 ), 0 ) ), 2, '0', STR_PAD_LEFT );
		$rgb[$key] = $value;
	}

	$hex = ( $trimmed ? '#' : '' ).implode( $rgb );
	return $hex;
}