The answer is no, Colombia could not elect a Green.
At least, that is the opinion of the Colombian electorate, who gave Uribist candidate Juan Manuel Santos a whopping 70% of the vote (or there abouts - reports are waffling between 67-69%), compared to the Green Party's opposition candidate Antanas Mockus, who garnered between 27-29% of the vote, the rest I'm assuming went to "none of the above."
There are several things to blame Mockus' loss on. Santos' popular position as Uribe's successor; lack of a strong Opposition vote; failure of Mockus' message to resonate among the bread-and-butter citizens of Colombia; his off-colour style. Whatever it was, Mockus clearly didn't stand a chance once all was said and done. Uribe's policies were simply too good to run against, and with some hints of Mockus sympathies towards the very-much hated Hugo Chavez of Venezuela, there was simply not enough love for Mockus to make any gains.
In fact, the Opposition vote was so bad that Mockus made no serious gains in the percentage of votes (I'd say popular vote, but that information isn't up yet). He went from 21.5%, to maybe 29% at the most - while the vote total from all Opposition parties would've added up to 36%. Not a very good sign.
But at the very least, the Greens showed themselves to be competitive. What effect this will have later on will be interesting to watch.