<?php

$dizi = "<b>domain1.net</b> -> <font color=red>Dolu</font><br
<b>domain2.com.tr</b> -> <font color=red>Dolu</font><br>
<b>domain3.com</b> -> <font color=green>Boş</font><br>
<b>domain4.com</b> -> <font color=red>Dolu</font><br>
<b>domain5.net</b> -> <font color=green>Boş</font><br>
<b>domain6.net</b> -> <font color=green>Boş</font><br>";

preg_match_all("#<b>(.*?)</b> -> <font color=green>Boş</font>#", $dizi , $domain);

for ( $i=0; $i<count($domain[1]); $i++){
echo $domain[1][$i]."<br>";
}
?>

// Çıktısı

domain3.com
domain5.net
domain6.net