The best time to visit Colombia will depend on your travel plans but the temperature varies little throughout the year. The temperature in Colombia does change with altitude, creating various climatic zones from hot lowlands to freezing Andean peaks, so you can experience completely different climates within a couple of hours of travel.
The most pleasant time to visit Colombia is in the dry season, between December and March or in July and August.
Apart from Bogota the majority of Colombia is HOT, and I mean HOT. So pack sun cream, hats, or even those fun mini fans because you are in for days of constant sweating.