Given the variety in landscapes in Colombia it depends on your itinerary what you should bring.

The Caribbean coast tends to be very hot, bring light clothes here, sun protection and insect repellent. Also the jungle is hot and humid. However in both areas it can cool off a bit a night so also bring a sweater.

In the mountains it can get pretty cold at night. Consider bringing a fleece jumper or jacket. If you like to do some hiking bring comfortable hiking shoes.

Bringing a jacket that protects you from rain and wind is always practical especially during rainy season. You can buy almost anything (in big cities) in Colombia but do bring your personal medicine-kit.