Really? The change of the color of the sky from white (daylight) to purple look a bit too abrupt for the sunset light. It looks as if you used a half-colored filter...
I did use a ND Soft Grad (Nisi) + polarizer which has very little color shift.. the transition was set to the horizon. and.. also, it was 2 hours before actual sunset, so the sun was low but not on the horizon.
maybe the filter, will have to be conscious to check in the future, here is another from last fall
actual sunset.