So far, I have:

$text = 'ju[color=1]s[/color]t a tes[color=2]t[/color]';
$text =~ s/
   (
      (?:
         (?:  \[ [^]]* \]  )*
         [^ []
      ){3}

      (?:  \[ [^]]* \]  )*
   )

   (?=[^ ])
/$1-/gx;
print($text, "\n");
# ju[color=1]s[/color]-t a tes[color=2]-t[/co-lor-]
[download]

It's not quite there yet, but I have to go.