The second-seeded Arruabarrena-Vecino dispatched the unseeded Colombian 6-3, 6-2 in 1 hour, 9 minutes, to claim her second crown in two years.
The 20-year-old's other championship also came in Colombia -- she won the Bogota event in 2012.
The 33-year-old veteran Castano had defeated rising Ukrainian Elina Svitolina in the semifinals to reach Sunday's title match.
Megyn Kelly: Santa Claus and Jesus are both white men
Wisconsin business offering 'therapeutic cuddling' forced to close