Athens is 7 hours ahead of Medellín
Best overlap: 8:00 AM to 11:00 AM in Medellín is 3:00 PM to 6:00 PM in Athens.
Shoot for 8:00 AM to 11:00 AM in Medellín; your Athens colleagues will see that as 3:00 PM to 6:00 PM. A 3-hour overlap gives you a workable daily window; aim for earlier in it to avoid clipping the end of someone's day. For part of the year the maths changes: Athens observes daylight saving, Medellín does not, shifting the overlap by an hour.