That was indeed unexpected.
Maybe the development staff made it cyan for balancing purposes. It could be they try to make every wu range segment and color group have an equal amount of varied items. I don't know this for sure, it's just my guess, but usually developers try to do something like this in other games.

Sometimes the White group has items that visually have rose/magenta accent colors in them. If it's necessary for it to not be part of the magenta/rose/red color groups, then perhaps it could belong in White?