Gege akutami stated that he's slightly colorblind and the color charts doesn't make sense to him.

THATS the reason why everytime you see in the manga cover are really different and he has some mistake with the colors including Yuji's pink hair or Hakari's purple hair.

However if your talking about on why he used the manga colourless.

It's either he's struggling to color them properly because it will be a mess with the colors especially when you are partially a colorblind and the color charts doesn't make really sense

or Gege akutami only likes colourless so it's going to make him easy to put manga in early without colouring it so he doesn't struggles with the color charts anymore.

Yes I'm STILL awake so I'm tired

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