Because ultimately, these characters will be used further as canon baselines for their elements. Right now, Chiara could have red in her color scheme, but if canonized, it then branches out into all Toa, Matoran, Turaga, etc of Lightning have red in their color scheme without any other basis than “Well, she could’ve.”
All other elements have additional colors from LEGO media, be it sets, MNOG, or otherwise. In this instance, we have a specific framework to deal with - Blue primary, white secondary. It’s too early to shut down the idea, but it needs some really strong arguments before it’d be allowed.
This argument actually bleeds into a discussion that hasn’t taken place yet - weapons. We don’t have any idea or description regarding weapons for those three. That would be another important question for Greg.
I’d like to encourage people to inquire with Greg about these questions. He still answers questions on the Boards, and it may be best for everyone to pitch their thoughts and suggestions for him to take a look at. It may not result in anything (see our post on this subject here), but it’s better than waiting.