At the Colón there are always some tickets reserved for high-ranking public officials who might want to attend a performance (government ministers, Supreme Court justices, a provincial governor who happens to be in Buenos Aires, etc.). Usually most of those tickets remain unused and can be bought a day or two before the performance at the ticket office on calle Tucumán. I would try my luck as soon as you get here, you'll probably be able to get a nice spot.
In my experience the dress code has become laxer over the years. Some 10 years ago every male at the Gran Abono (1st performance) would show up on a dinner suit/tuxedo and women would wear long dresses. Last year the attire was still formal but I only saw two or three men wearing tuxs, the rest were on normal suits and tie. Still, you're expected to dress formally unless you buy a ticket for paraíso (the closest to the roof, hence the name) where there's neither such etiquette nor seats - you have to remain upstanding during the whole performance.