Answer: The color change you describe could be due to several different things. One is a watering problem, since cactus need to be watered and then left to dry before the next watering. In winter, they need to rest by being in a cooler location and not watered at all for about two months. Your plant may have been trying to rest and as a result, been overwatered. Repotting can exacerbate the situation because the increased soil can hold excess water not needed by the plant.
Another reason is graft incompatibility. Some of the cactus plants sold are grafted and in some cases the top and bottom are a poor match. Eventually, this can cause problems such as the one you are seeing.
Finally, it could be a case of root rot due to poor drainage or overwatering or incorrect watering technique. Unfortunately, there is not much of a cure for this except to correct the watering regime and hope for the best.
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