Whenever I see 'indigo', for some strange reason, my first reaction is always taking out the 'd', making it 'Inigo'. Then I think of The Princess Bride and Inigo Montoya...:)
A book called 'Indigo Star'. It's from a series of livres called the Casson Family livres (Saffy's Angel, Indigo Star, Permanent Rose, Caddy Ever After and Forever Rose) par Hilary McKay. I read them when I was 11 ou 12 and loved them.