In the 80's I owned a couple of Yamaha Natural Sound components which were quite good. This is a corporation that is fanatical about musical instruments as well, so you think they may know something about sound, eh? Would anyone put their money on a small shop to test such things - and likely have some data to back it up? It's entirely possible that it's a marketing line, but often big, focused companies that want a reputation as pushing the limit on performance are not going to invite ridicule by putting fluff in with their performance criteria.
Obviously not an effect of color, but finish. Again, in a state of the art sound room for testing, I would not be surprised if there were differences heard based on the finish. It is incredible how fine the resolution and tonality can change based upon seemingly insignificant changes. The owner would not be expected to hear such things, imo. That in no way means it cannot be an influence in a testing facility. Until you have done the testing, you are in no position to say they are wrong. You may disbelieve, but that says as much about you as the company.
My guess is that the finish as heard in the sound room would influence the reflected signals (reflected off the cabinet? perhaps), allowing slightly more decay than a flat finish, and would be understood as allowing more of the signal to be heard. You would have to be in a proper sound testing facility to have a chance to hear such a thing.
I do not mind taking the minority position on many matters in audio. As in life/science, I find that the popular opinion is often wrong. It's very easy to mock, but it's not such a laughing matter when it can be backed up. Can Yamaha's assertion be backed up? Maybe, maybe not. It certainly won't be resolved by you - unless you have your own testing facility with state of the art room and equipment.
We all have our skepticism triggered at different levels. I find it absurd that HiFi enthusiasts who claim they want superior sound would not understand/at least try aftermarket cabling. When "good enough", lowest common denominator, i.e. all caps sound the same, audio is used to assess an assault on SOTA, you do not understand the things that are done to achieve it, and the outcome. Notice the alternative was not discussed, that is, the alternative speaker finish/coating/material. A speaker like a Vandersteen - would anyone doubt that an identical speaker covered in a fabric would sound different under testing than one with a glossy finish? I certainly would not mock that possibility.
The range of products certainly does run the gamut, from Tekton, which puts a dozen drivers for midrange into an MDF cabinet, to Yamaha, which fiercely limits the drivers, particular cabinet material, and even finish! Something for everyone!
