/jp/ - Otaku Culture

Upon researching it further, several Japanese sources point to a certain "store bonus" which had the very first color depiction of Reisen as the basis of her ocean colored hair. Just as a background, whenever some new book gets released in Japan, different stores offer different exclusive extra items related to the work in order to attract customers.

https://dic.pixiv.net/a/レイセン
>底巻のアニメイト特典により、水色の髪を始めとした公式のカラーが判明。
>Reisen's blue hair color was established from an official color illustration that was included in copies of Bougetsushou Volume 3 purchased from animate [an otaku goods shop much like melonbooks]

https://tohotoho2.exblog.jp/amp/12445937/
>おお、レイセンのカラー絵がついにキターーーーーーーーー！！
>finally a colored Reisen illustration!
>しおりシートそのものも、依姫・豊姫・レイセンという、儚月抄新キャラが勢揃い
>We have Yorihime, Toyohime, and Reisen on a bookmark [the illustration referred to above]

https://verfe.exblog.jp/amp/9053320/
>アニメイト→綿月姉妹＋レイセンの栞シート
>from animate: Moon sisters + Reisen bookmark

Pictured is a fanart on pixiv trying to get people to draw Reisen with blue hair, citing the extra item from animate's edition of Bougetsushou Volume 3.

What does that bookmark look like? How are we sure it's official? Well, this took a while to find but...