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.

collection is in other words holding or gathering some thing while collocation is arrangements of words or some thing side by side collection of items