Tantilla melanocephala is very similar in appearance to Tantilla reticulata, but that species would have lighter markings in the neck area instead.
Twan Leenders, biologist and author of the Guide to the Amphibians and Reptiles of Costa Rica and some other books on reptiles and amphibians in Costa Rica, helped me identifying this species.
I met Twan during my visit to the Biological Station Rara Avis (now an Eco Lodge) in the Braulio Carrillo National Park in Costa Rica in 2003 and ever since I sometimes ask his assistance in identifying reptiles and amphibians that I have seen.

Dapa area, Valle del Cauca, Colombia, 11 June 2016