Not sure if everyone knows this or not, but Kirby and Lee designed the FF’s powers to reflect the four elements. Johnny and Ben are obvious, fire and earth, respectively. Sue makes sense once you think about it, invisible air. It’s where Reed’s powers come in, that make people raise an eyebrow. He always wears blue, in fact his is the one costume that has hardly ever changed; Johnny wore red for a while, Sue has worn various sexy versions of her uniform, and Ben has gone from trunks, to pants and boots, to full body suit and back, but Reed stays the course, thus cementing his role of water, blue and flowing.