I would have to agree with you, Alan, on pretty much all points. Amongst those (few?) of us who truly value originality, it might have some effect on value. Even there, though, that's likely only true IF there is documentation of the original color from the factory, evidenced either by a BMIHT build certificate or, on Triumphs from around 1964 on, the data on the commission plate showing original color and trim codes.
I love Signal Red. Unfortunately, it sometimes seems that, out of the 80,000 or so sidescreen TRs built, approximately 112% of them were painted Signal Red.
Black, especially with a red interior, is simply stunning in my opinion.
There are far too many sidescreen TRs with tan interiors, again in my opinion.
Frankly, this all really boils down to the classic debate of "are you restoring the car for resale value" or "are you restoring the car for YOU to enjoy"?
I probably wouldn't do it myself, but I've seen drop-dead-gorgeous TRs in such colors as Metallic Blues and "Champagne" metallic hues.
Figure, for sake of argument, that it cost around $4000 for a show-winning, thorough paint job. I rather doubt that such a job in a nonstock color would devalue the car by that much. It can always be changed again by a future owner, right?
Of course, then there's the early TR2 colors of Olive Yellow, Geranium, Ice Blue...colors that very, very few people seemed to like then or now (except me...I love 'em)!
Go with what you like!