When I bought my first replacement set of red line Michelin's for my TR6 I had the red stripes turned inward, which is something I would not do now.
Things and tastes vary. I remember as a young man when the TR3's were out and virtually all ran black walls. I also referenced "The Triump TRs, a collectors guide" by Graham Robson, and other references, all the photos of the TR3 and TR4 were with black wall tires.
That said, on your TR3 I think white walls add that little extra special look of class to the car over the first black wall picture you posted. When I asked my wife, after telling her white walls were not original, she said, "So what, the white walls look better".
So there you go.