I bought a Mandevilla plant that had red flowers. They are now re-blooming but are white. What's up with that?
Hm-m-m, very interesting. Could be that you have one of those varieties that blooms pale color, and then the blooms gradually turn dark red as they age. So when you bought it, it had all older blooms, not the blooms are new. They might turn color. Another thought is that something the grower used to force growth/bloom for sale affected the color of the flowers, now you are seeing a more natural hue. If you can get the name of the cultivar and/or the grower, you might be able to get more information. Here's an article that has general information about mandevilla: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm