For the mint green thing, maybe it's meant to be exactly as you said - the team's official colours are the exact shades of the rainbow, but all the team members aren't, so

As for chosing white above all colours, I don't think it was chosen because they wanted a white there, but because Ayaka embodies what the colour white represents, which I've listed below:

1. She has a pure and innocent image + personality, like the colour itself.
2. She started out literally as a fresh, clean sheet of paper with no experience and natural talent at all (especially as compared to Mayuka and Riku), and even though she got into NiziU already, she's still learning and committed to putting in the hard work and effort like she did during Nizi Project, something like how ~5-7 k-pop idols would say "we'll continue to have the rookie mindset and work hard", you know? Her being assigned white can therefore can be said to be a reflection of her past roots and current continuous positive work ethic.