umm.. sorry to say this but 'platano' is not banana.
Its plantain.
Different fruit, same family.
Bananas are green, sourish and hard of texture when they sprout, then turn yellow, sweet and soft on the inside when they mature (turn yellow).
Plantains are 1/3rd bigger in size than bananas, are always sourish and have a hard texture to them and when they mature the outer skin usually stays green (some do turn yellowish but its a sickly yellow) but the inside still remains sourish and rather hard.
http://en.wikipedia.org/wiki/Plantain