I don't know about Colombia, but here in Peru when they say formal, they mean formal, like ball gown and tuxedo formal.
Also it is winter here, so a spaghetti strap dress would be way too cold. However, I don't know what the temps are like in Colombia, so I would double check that.
I just remembered that I'm mad at the country of Colombia right now!! lol
A priest from my area went to Colombia to visit his family and now he's stuck there. Apparently he shares a name with someone who is a notorious drug lord and ICE and Homeland Security won't let him come back to the US until the matter is resolved. The last we heard is that if he comes back by the end of the year it will be a miracle.