According to the Rainbow Brite Colouring Book, her full name is...

Indigo Doll

I find it fitting. What about you?

That's really cute ^_^ I'd always wondered what her full name was.

It's quite interesting. I had always wondered why she never had a two word name as the others did in the show.

Very weird...

This is a misprint. Indigo's full name is Indigo, as all of the stories and her box say. This coloring book is British, and all of the British books have two other errors. Those say that Saucy and Encore are Lala Orange's and Indigo's head sprites, but they are not. OJ and Hammy are. The other Color Kids have two parts to their names because most of their colors don't make sense as names alone. Red and violet are names, but the additions of Butler and Shy make the said personalities come out completely and truly show what Red Butler and Shy Violet are about. Red Butler's name especially screams his character traits. Indigo stands on its own as a reasonable name, and does not need a description because it's a very unique color that is self-explanatory. All of these are reasons why Indigo is the only Color Kid with a name comprised of one word.