Usually EGGD but not always. Its the country the aircraft is registered in, which is not always the airline's home.
Examples abound. Singapore Airlines for a time operated a 707 registered ET-ACQ, it belonged to Ethiopian and was on lease. Philippine Airlines 747-200s were all US registered if I remember correctly, as they were owned by a leasing company in the States. CityFlyer at Gatwick operated a Brit Air ATR 42 for several months with a French registration.
I believe in Colombia's case, an X is added to the registration to show that the aircraft is leased from abroad.
Just as ships are often registered under the Liberian flag regardless of which nationality they are, there are sometimes financial advantages to registering the aircraft in some foreign state, under a "flag of convenience". Sometimes the leasing company, the outright owner, will require it to be registered in their home country. Sometimes its a short term lease and the paperwork/expense involved in transferring ownership isn't worth the effort.
Regards - Musang