This is a very broad request. Here are a number of considerations.
First, from one perspective, style, fashion, and fad are all related. If something is in style or fashion, it can be a fad. The only difference is that fads come and go, where as style and fashion are more enduring.
In light of this, I would say that bright colors are in fashion and style this summer. If you look at the runways and the catalogues, bright colors are fashionable this spring. In particular, bright pants and dresses have been emphasized by many of the top fashion companies. I would not say that this is a fad, because there will always be a place for bright colors. In other words, bright colors have passed the test of time.
Now when it comes to fads, it is hard to tell what is a fad currently, because only time will tell. However, when I was growing up, parachute pants were a fad. They were popular and they went away rather quickly, never to come back.