Some heather cultivars are specifically selected because their foliage changes color in winter. I have one that turns a bright combination of orange and gold, so foliage color change in winter isn’t necessarily an issue. Do you know the cultivar name?

However, we don’t know enough info to judge whether yours is in trouble. Please add a photo of your plant in the comments section below (give it a bit of time for the upload to change from grayed out to full color before hitting submit) and tell us the city and state or country where you are growing it. How often are you watering it, what is the potting soil like, and how much light is it getting? In my experience, heathers like well-drained, acid soil and don’t tolerate wet feet and like quite a bit of sun. If your winters are cool, they will need far less water than in summer. A plant in a pot has roots exposed to a lot more swings in temperature than one in the ground, and so if you get temperatures that are too cold, that may outright kill them. In the ground they are OK in my area of central NH, but I couldn’t keep one alive in a pot in winter since it would be too cold.

Sorry responses are slow. This is a forum with very few visitors, so I often don’t check it. Hopefully others will weigh in with advice.