Hmm is this a leading point for Azusa's friend to join as a back up magical girl?
It looks like you can only become a magical girl if your name is already a bad pun with a colour in it. Minami is a compass point (south), so she doesn't qualify.
(checks dictionary again)
In fact... "nami" even means "ordinary." That may not be accidental.
Doesnt Nami mean wave?
It can also mean that. In fact, "Minami" (深波) is written with the character for "wave", but the puns in the other names are based on the pronunciation only, not the kanji, so that's probably irrelevant.
By the way, in case anyone was wondering:
Gunjou Azusa - "gunjou" = "ultramarine", AZUsa -> AZUrite
Akaishi Harumi - "akai" = "red", HaRUMI - > RUBY
Minamiya Minami - "nami" = "ordinary" (maybe?)