给定一个颜色值，如何把它转换成一个深一点的值

给定一个颜色值，怎么把它转换成一个深一点的值？
a = "#333333"
需要这个函数：把a的色彩变深一点，比如a="#222222"

------解决思路----------------------

$a = "#333333";<br />$rgb = unpack('C*', pack('H*', substr($a, 1)));<br />$hsb = call_user_func_array('rgb2hsb', $rgb);<br />$hsb[2] *= 0.67;<br />$rgb = call_user_func_array('hsb2rgb', $hsb);<br />echo '#' . join('', array_map('dechex', $rgb));<br /><br /><br />function rgb2hsb($rgbR, $rgbG, $rgbB) {<br />    $rgbR &= 255;<br />    $rgbG &= 255;<br />    $rgbB &= 255;<br />    $rgb = array($rgbR, $rgbG, $rgbB );<br />    sort($rgb);<br />    $max = $rgb[2];<br />    $min = $rgb[0];<br /> <br />    $hsbB = $max / 255.0;<br />    $hsbS = $max == 0 ? 0 : ($max - $min) / $max;<br /> <br />    $hsbH = 0;<br />    if ($max == $rgbR && $rgbG >= $rgbB) {<br />        $hsbH = ($rgbG - $rgbB) * 60 / (($m = $max - $min) ? $m : 1);<br />    } else if ($max == $rgbR && $rgbG < $rgbB) {<br />        $hsbH = ($rgbG - $rgbB) * 60 / ($max - $min) + 360;<br />    } else if ($max == $rgbG) {<br />        $hsbH = ($rgbB - $rgbR) * 60 / ($max - $min) + 120;<br />    } else if ($max == $rgbB) {<br />        $hsbH = ($rgbR - $rgbG) * 60 / ($max - $min) + 240;<br />    }<br /> <br />    return array( $hsbH, $hsbS, $hsbB );<br />}<br /> <br />function hsb2rgb($h, $s, $v) {<br />    $r = $g = $b = 0;<br />    $i = ($h / 60) % 6;<br />    $f = ($h / 60) - $i;<br />    $p = $v * (1 - $s);<br />    $q = $v * (1 - $f * $s);<br />    $t = $v * (1 - (1 - $f) * $s);<br />    switch ($i) {<br />    case 0:<br />        $r = $v;<br />        $g = $t;<br />        $b = $p;<br />        break;<br />    case 1:<br />        $r = $q;<br />        $g = $v;<br />        $b = $p;<br />        break;<br />    case 2:<br />        $r = $p;<br />        $g = $v;<br />        $b = $t;<br />        break;<br />    case 3:<br />        $r = $p;<br />        $g = $q;<br />        $b = $v;<br />        break;<br />    case 4:<br />        $r = $t;<br />        $g = $p;<br />        $b = $v;<br />        break;<br />    case 5:<br />        $r = $v;<br />        $g = $p;<br />        $b = $q;<br />        break;<br />    default:<br />        break;<br />    }<br />    return array( intval($r * 255), intval($g * 255), intval($b * 255) );<br />}<br />

#222222