buy it in the US and use it in Colombia?

A cellular model will not work.

A GPS model works anywhere the paired iPhone works.

Warranty is Not international, so if your Dad would have to send the Apple Watch back to you for warranty service.

ECG will not work, as Colombia has not approved the app yet.

You may want to review the Apple Watch country specific feature list, as some things you assume will work, may not be approved in Colombia

Apple Watch Feature Availability by country

https://www.apple.com/watchos/feature-availability

But as julieda suggests, buying in the country of use is the safest way to get all the functionality offered by that country.

This will be of interest to you. My recommendation would be to purchase the watch in the country where you intend to use it. In this Columbia........https://www.apple.com/watch/cellular/