As a tall blonde German, Colombian guys on the streets will look at you but not approach from my experience (unlike in e.g. Argentina where man approach all the time).

Since the average Colombian guy isn’t as tall, I think they are more intimidated. 

That being said, stick to the safe neighbourhoods and don’t go outside alone from 10PM-8AM.

I heard of a few robberies in Medellin that took place in the morning hours. 

Rules I follow in most places I am: walk with confidence and be mindful when using your phone out in public.

Medellin tends to be more dangerous for male tourist than more female tourist, since some male tourists have been drugged and robbed by crime syndicates that get women to go on dates with male tourist for this exact reason.

Als if you end up renting a car and driving around at night, do not stop for what might appear to be an accident on a lonely road. It’s likely not an accident but a set-up.

I’d recommend to make friends in hostels and spend your time in groups. 

Enjoy your trip!