$rand = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'];
$random_colour = '#' . $rand[rand(0,15)].$rand[rand(0,15)].$rand[rand(0,15)].$rand[rand(0,15)].$rand[rand(0,15)].$rand[rand(0,15)];
Also, an alternate one-liner:
$random_colour = '#' . strtoupper(dechex(rand(0,10000000)));