I have now added the tag of "artistic error" due to Yuyuko's hat,which has to be involved with the fact it looks like almost similar to Chen's green beret-cap from behind,being the canonically wrong and seeries-discordant ("Ran-chama) colour.

JMI said:

I have now added the tag of "artistic error" due to Yuyuko's hat,which has to be involved with the fact it looks like almost similar to Chen's green beret-cap from behind,being the canonically wrong and seeries-discordant ("Ran-chama) colour.

While Yuyuko's mob cap looks a little more turquoise than usual, it's still definitely a shade of blue, as it should be. Chen's mob cap is a more definite green (that is, with a significantly higher Y-value than the color of Yuyuko's cap in this image), though the exact shade varies. This seems to be more a matter of artistic license than artistic error.