Definition of "differentiate": Difference "in" and difference "between"

"Difference in" is usually used to refer to a change in one thing, instead of noting the dissimilarity between two things. "I noticed a difference in the way you play piano now. It's so much softer than it used to be." "There's a difference in my attitude. I am happy at last."

"In" is also used when you are referring to the difference between two things by pointing out the difference itself (i.e. the ONE thing that's different). Consider these two ways of saying the same thing: "There's a difference in the way you use 'cold' and 'frigid'," versus "There's a difference between 'cold' and 'frigid'."

So use "in" with a single object, and "between" with a pair of objects.