I looked around a bit, and it looks like Plum Mist is more purple, whereas Iris Mist was more silver. Both are great colors.
What’s really interesting is that some of the least popular colors of cars back in the day were the most beautiful. Buick had a color called Champagne Mist in the mid sixties that was absolutely beautiful. Very few cars were painted that color, I recently worked on a 65 Riviera that was painted that color, beautiful car.