I don't see it so much as Colombo being opportunistic as much as him playing it safe. He could stay loyal and be a part of war he may have lost and loose money and his life or reveal the plot and prevent anything from happening.
On the other hand, he couldn't be 100% sure he wouldn't have been killed later. I mean, Gambino and Lucchese could have thought "if Colombo betrayed his boss Magliocco to them, then he could later betray them to somebody else." Once a traitor, always a traitor. Seriously, I don't know what would I have done in Colombo's place. Probably just turned everybody to the FBI