To further explain Sato-san's hypothesis that the colour coating makes a difference, they talked about the process of "alumite" in the video.
http://www.nacl.co.jp/en/alumite/
While the Japanese refers to this as alumite, the closest to our English understanding is the process of anodizing:
https://en.wikipedia.org/wiki/Anodizing
TL;DR - it's a process which artificially increases the oxide ("rust" - if you must) film on the surface of the metal in order to protect the bare metal underneath. Metals such as aluminium has a natural oxide film on the surface but it is still common to anodize it further to increase this film to a thicker thickness to increase protection and/or to use the pores introduced in the process to inject colour (such as in the case of the black version). As said the anodizing process is literally introducing oxide ("rust") to surface of the metal and thus decreases its conductivity and purity, and in the case of the silver vs black of the ZX300 the anodizing in the black version is much thicker in order to take on the colour, thus with the internals being the same and the chassis ground property being different (the thicker oxide film of the black version causes the chassis ground to have a slightly higher resistance than the silver version, thus venturing off from the ideal ground that the chassis is suppose to provide), they speculate that is the cause of the sound difference.
If you believe in purity of copper cables affecting the sound (which many cable believer will buy into straight away - that's why they insist on OFC cables, no?), this doesn't seem like that far fetch of a hypothesis.