agree with the 2 beautiful color beams. As for the question how many times they can fire it. Ultraman fired 3 times at Aboras with his beam. I sure that other ultras have fired their beams more than once i just cant think of any other instance other than the Aboras one.
In some episodes, the ultra did fire his specium more than once at a monster but has little or
no effect on it. This is because the ultra fired his specium too prematurely at the monster as
it is still not weakened enough or the monster is just too strong. Some monsters do absorb
the specium ray and this a waste of precious energy for the ultra. To effectively destroy the
monster, the ultra can fires his specium when the monster is in the most weakened state
or when it is caught with its guard down.