Do I need an weight distributing hitch for my Colorado?

Even if it sits OK sitting still, when you hit the wrong dip or wavy "wow" in the road, the weight distributing hitch will help keep the truck level instead of doing a porpoise.
When you're moving all sorts of dynamics are in play.

Dealer salesmen are rarely towing experts. Most don't really know much about cars at all. They do know how to sound confident and knowledgeable and sell cars.
You did the right thing asking your question here instead of buying whatever line of... uh... rose petals the dealer tells you.