function random_color_part() {
    return str_pad( dechex( mt_rand( 0, 255 ) ), 2, '0', STR_PAD_LEFT);
}

function random_color() {
    return random_color_part() . random_color_part() . random_color_part();
}

echo random_color();

function rand_color() {
    return '#' . str_pad(dechex(mt_rand(0, 0xFFFFFF)), 6, '0', STR_PAD_LEFT);
}

function rand_color() {
    return sprintf('#%06X', mt_rand(0, 0xFFFFFF));
}

$color = substr(md5(rand()), 0, 6);

$rand = dechex(rand(0x000000, 0xFFFFFF));
echo('#' . $rand);

$color = sprintf("#%06x",rand(0,16777215));

echo '#' . substr(str_shuffle('ABCDEF0123456789'), 0, 6);

$colors = array();
while (true) {
   $color          = substr(str_shuffle('ABCDEF0123456789'), 0, 6);
   $colors[$color] = '#' . $color;
   if ( count($colors) == 10000 ) {
      echo implode(PHP_EOL, $colors);
      break;
   }
}

function randomHex() {
   $chars = 'ABCDEF0123456789';
   $color = '#';
   for ( $i = 0; $i < 6; $i++ ) {
      $color .= $chars[rand(0, strlen($chars) - 1)];
   }
   return $color;
}

echo randomHex();

$hex_string = bin2hex(openssl_random_pseudo_bytes(3));

<?php echo 'rgba('.rand(0,255).', '.rand(0,255).', '.rand(0,255).', 0.73)'; ?>

$color= substr(str_shuffle('AABBCCDDEEFF00112233445566778899'), 0, 6);