Cel-shaded games should be as colorful as possible.
I wholeheartedly disagree with this. The whole point of NPR is simply present the data (in this case game imagery) in a fashion that the presenter feels most suitable to convey what they're trying present.
Using a toon-shader does not inherently mean that bright, poppy colors are automatically a must. While I too prefer bright colorful games, I don't believe they're answer to every game out there, even if they rely on various NPR techniques...