Well when Sydney knows the killers, she goes into the closet or something. There she changes into the "scream" costume. And with the costume she has black shoes on. But when she stabs Billy and they fall her shoes are now blue and white sneakers! Those are some pretty cool shoes!
I'd also like to mention that she originally was not wearing tennis shoes at the party...she was wearing heels. Yeah, I keep extra shoes at my friends house in case my boyfriend turns into a serial killer too:-)
I just watched the movie and when it got to that part I thought it was weird to about the shoes but if you watch it closely it shows her coming out and then it shows the shoes they must have aimed the camera from a different angle because those are billy's shoes, because right after he runs into the living room, they show his shoes.