I think the tone issue dates back to the Tim Burton Batman film. It was a gamble at the time, going dark when much of the audience's identity of Batman traced back to the candy-colored Adam West Batman series, and the gamble paid off. I believe that's where the thinking became ingrained. When they did the first 'Flash' TV show, it struck me as ludicrous that they would darken his red costume and go with deep shadows, but it seemed apparent that the Batman movie was the template -- even though the Flash of the comics was one of the more brightly-colored and least mysterious characters out there. The idea of different super-hero worlds employing their own distinct tones didn't seem to be on the radar.