Certified GKH Gardening Expert
It could be a lack of nutrients in the soil, or that the plant is getting more sun than it did wherever you got it. Both of these can influence the vividness of color on the flower. It may also be that the original plant was grafted and the scion died and was replaced by the rootstock, which produces a different color. The only other thing I can think of if none of these fit your situation would be that the plant was mislabeled.