Each year about this time of the calendar year this happens to a handful of hoya plants. The leaves turn a nice blush of pink or red or any color in between. Some collectors and growers claim that providing bright light will give the same affect but I am not sure that is the reason. Nothing has changed for my plants that are happily growing under artificial lights and these few have made a drastic change on some of their foliage. I find it amazing and stunning each year that this happens!