These are associated with noble metals (24-carat Gold, Silver, Platinum) or with Titanium gas which gives them iridescent colors. As for the other colors, they are colored films that allow the desired light frequency to radiate.
In any case, each bowl is completely unique in the play of its spirals, both in its fundamental note and in its struck note. Some vessels emit sound frequencies of great power, and other bowls dance on frequencies of incredible subtlety... And there is not one "better" than the other.