I am stupid about this. One I know someone, they just look like themselves to me, and I actually don't notice most changes in appearance, unless their hair suddenly becomes, oh, hot pink, or they suddenly grow a beard. And I probably would say just that, and add something like, "I thought it was the dress. Have I seen that before? I like how it looks on you."
If the response is, "Yes, a thousand times, and you were with me when I bought it!"
I would probably answer, "I bet I say that every single time I see it! It is a keeper."