Dijon mustard is delicious. It has a nice, tangy flavor that adds a lot of taste to potato salad. However, this is a classic potato salad recipe. And, back in the day, the sweet, yellow mustard is what everyone used. So, if you want to keep this classic, use yellow mustard. Plus, yellow mustard add more of a yellow coloring to the potato salad, which is the color you expect to see with a classic potato salad.