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...I don't think I've ever seen a bike that had a contrasting head tube that didn't follow through with the traditional seat tube panel.
So I'm not sure how it will strike me visually...your idea of a head tube with the same panel on the downtube, rather than the seat tube.
Otherwise, in terms of color, any of the eggshell whites are a little yellower than stark refrigerator white. And there is a cornucopia of yellows, from light to dark, that would probably work.
If it's a really dark plum, even black would probably work out OK. But then you have to worry about how the decals will stand out on that background.