OK.
Look at a Rothko canvas, particularly a later one. There will likely be a few closely-valued hues that make up the color field. There are subtle variations between the areas of color, but that is not the same as repetition & variation of a theme (color, shape, size) on mathematical principles (or the illusion of such). The latter is one way to define "serial." That's usually the definition applied to minimal art.
Minimal music - Reich, early Glass, Gibson - adheres mostly to that second principle. It's more in keeping with the visual art of say, Sol Lewitt or Carl Andre. Feldman's music is analogous to what I'm saying about Rothko.