The series is inspired by Japanese culture and mythology. Wearing kimonos is as common as wearing jeans in the US.
Note: Kid Oro also wore a kimono.
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and the Ama orphans
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When have Mitsuki's eyes been white? I've always seen them as yellow in both the colored Gaiden pages and now SP promo material.
Honestly, Bortuo and Mitsuki have the most confusing eye colors. No one seems to know what they are, not even Kishi.
You forgot the fact that certain clans regularly wear Kiminos such as Hyuga, Kaguya and more or less the Uzumaki are the only clans that I can think of so it's not too common based on this series. Keep in mind most of the clans I listed are extinct.
However I do agree with you in some aspect that Mitsuki have certain elements that Orochimaru have when his clan (If there's ever one) most likely follows their dynasty Chinese/Japaneese tradition to wear their proper clothing which is Kimino as one of their culture.
Though there are some misconception with your belief of what Mitsuki truly wearing may not be Orochimaru related at all. I'm talking about this one right here:
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You see where I highlighted in the image that shows Mitsuki having long loose white sleeves is almost complete identical to the Ootsutski clan that toneri is wearing. If you look at the Blue in the other half that Mitsuki is wearing, it's possible that it's a padded across his other clothes. Whether it's a separate clothing or not it does not really matter when his Kimino fits the resemblence of the Ootsutski/Hyuga clan since they are are strongly related, which is more plausible than Orochimaru.
Now to answer your last remaing bit about the eyes now.
MP
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MS
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Promo
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You see the difference??? Both Gaiden chapter and Promo have different colors of Mitsuki eyes. In mangapanda version, his eyes were silver white compare to ***********, his eyes is yellow. I believe this is the only time we've seen Mitsuki in color, unless theres more that I probably have not realize, then please share with me.
Indra has posted the updated Mitsuki from the promo but this time it's a clear full body pose and his eyes are white silver and it's official color scan according to the source from Rai in Naruto Forums.
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But like I said before, even if his eyes is yellow, regardless it's clear that Mitsuki originated from the Ootsutski clan but I do believe he's a nemesis of Orochimaru and he's being product of an experiment.