Complex sentence: Colombia does not collect racial data in its census; however, according to external sources, whites make up 20% to 25% of Colombia's population, and thus constitute the second largest ethnic group of country, only after mestizos.
Split into simpler sentences: Colombia does not have a racial census, however according to external sources, whites make up 20% to 25% of Colombia's population. For this reason, they constitute the second largest ethnic group of country, only after mestizos.