They actually have no overlap in meaning.
A collocation is a phrase made up of words that are always, or very often, used together.
A collection is a group of things. Usually they have something in common (like a stamp collection). Collection also means going to a place to get something and to take it away from there. For example, if you order food from a restaurant, you could choose delivery or collection. Collection means that you will go there and take the food away yourself.