I teach 5-6 times a week in the white building to the right of the brown and green Torre Colon. It is Number 1, Castellana, a prestigious address indeed and home to the Madrid office of Banque de Credit Agricole.
Christopher Columbus's stature is dwarfed by the Torre Colon behind it. Torre Colon was one of the early Madrid Skyscrapers and for a while in the early to mid 70s was the tallest building in Madrid. The green top was a later addition, apparently added when it's pre-eminence in altitude came under threat!