In the case of @Kimbridgeman that could be a possibility, especially if this is the first time he has seen this change of color of his 4 desert roses. Hope he stops by again to let us know if that could be a possibility.
I'm guessing too it could be temperature related if all 4 purple exhibit the same shift in color. Like I would notice my two King Purple shift color at the same time:
In either case (fertilizer or temperature) the change in color is normally temporary and he could see all four bloom purple again once we start the late spring or summer.
