Yes, sometimes the flowers will change color. There are lots of Monarda hybrids and hybrids by nature have unpredictable reproduction, often displaying traits from their genetic heritage. So don’t be surprised if a light pink or purple one is red next year. Bee balm can also cross-pollinate so yours may be getting pollinated by some neighboring bee balm flowers in different colors. If your experience is like mine, one color will become dominant over the years. In my case, I love the deep purple ones but I have ended up with a garden of red flowers. I suspect the key is to love whatever shows up.