Seven of Seven/Headscratchers

Why is "yellow-green" one of the Nanas' colors?
The Nanas' colors are supposed to be the seven colors of the rainbow, right? Since when does that include "yellow-green", which is a tertiary color? What happened to indigo?
 * Since when is indigo really distinct from violet? Everyone knows that they futzed with the classification to get it up to the religiously significant number of seven, the makers of this series simply chose a different "filler" color than is conventional. Yellow-green is in the rainbow too, its that spot between yellow and green.
 * In Japan, kimidori is one of the seven colours of the rainbow, and indigo isn't. (The divisions are arbitrary, after all.) It's that simple.

What aspect of Nana does blue Nana represent?
My guess, blue Nana is her good side! Think, she's the only one who could beat Jamanana, knew how to calm down Nanarin, and because she's Nana's good side, she's not very assertive, hence why the others are able to push her around. Also, this would explain why she seems to be on a different wavelength than the others, who have more rounded personalities. The reason she is so much like the regular Nana is that Nana is a good person at heart.
 * Wasn't blue Nana just supposed to be the original Nana?