Noel is Blue, not Indigo.
Noel, the princess of the Arctic Ocean, uses the Aiiro Pearl to transform into Aiiro Pearl Voice. But how should "aiiro" be translated? If you look up aiiro in a dictionary, you will see one definition: "indigo." And I'm here to tell you that most dictionaries are wrong in this respect. Well, not exactly wrong, per say, but the point is that the dictionary only tells half the story. This is a common frustration when translating color words between different languages. What it all boils down to, in the end, is that aiiro is a Japanese word used to refer to a broad spectrum of color that English-speakers would label separately as "blue," "navy blue," "dark purple," "indigo," etc. And, above all else, trust what your eyes tell you: Noel is blue. Period.
Below is a lengthier version of my explanation for why you can't always trust that Japanese-to-English dictionary.
Color words: they're often not what you think.
I'm a linguistics minor, and one thing that's always fascinated me is the way that color words often do NOT translate well between different languages. Japanese to English is a good example. Originally, Japanese had much fewer color words than English. In fact, if you go back and look at classical Japanese literature, only one single word, aoi, was used to refer to most shades of blue and green. Today, modern Japanese has a different word, midori, which means "green," and aoi is supposed to mean "blue."
However, that's not always the way that it works. A dictionary will tell you:
Aoi means blue.
Midori means green.
And yet, the following are still common uses of aoi in the Japanese language:
Sora ga aoi == The sky is blue.
Mori ga aoi == The forest is green.
Shingo ga aoi == The traffic light is green.
Banana ga aosugiru. == This banana is too green.
Yes, you guessed it. Even though a dictionary will only tell you that aoi means "blue," the truth is that Japanese speakers still use aoi to refer to many things that would translate into English as "green."
So let's say that you're a translator who relies a bit too heavily on your Japanese-to-English dictionary. You see a picture of a green banana. The caption of this picture says, in Japanese, aoi banana. You need to translate the caption of that picture. So you look up aoi in your dictionary, and you get "blue." In fact, you look up aoi in several dictionaries, and you only see "blue" listed as the definition. Aha! you think to yourself, the caption must translate as BLUE BANANA!!!
Except that any native Japanese speaker would understand that in this instance, aoi is supposed to mean "green," a leftover usage from the language's more archaic past. And, most importantly, THE BANANA ITSELF IS GREEN. Hence, the only reasonable translation would be GREEN BANANA.
The same thing is happening with aiiro in Pichi Pichi Pitch. Noel uses the Aiiro Pearl to transform into Aiiro Pearl Voice. If you look up aiiro in a dictionary, you will find "indigo," probably listed as the only definition. So, you think to yourself, Noel must be "Indigo Pearl Voice."
Case closed, right? Absolutely not. The simple truth is, as was previously demonstrated with our example of the "blue banana," in the case of color words, dictionaries usually fail to explain how those words are actually used and understood among native Japanese speakers.
Aiiro is a word that encompasses shades of bright blue, dark blue, and dark purple. It also includes the blend of purple and blue that we would call "indigo." In English, we would use separate words for all of these different colors, but in Japanese, they can all be described with one single word. Now, this is difficult from a translator's point of view, because aiiro must be translated as a different English word every time, depending on which color it is being applied to.
For the record, I did some impromptu experiments with the word aiiro when I was living with a host family in Japan. One time, I asked my host mom to point out things to me that she would call aiiro. She showed me a pair of my host sister's dark blue jeans. I asked a friend of mine if she could show me something aiiro, and she showed me an oven mitt that I would describe as navy blue. There's also a common phrase in Japanese, aiiro no umi, which means "the deep blue sea."
So, there you have it. Aiiro can be applied to colors that, in English, would be labeled as "blue," "deep blue," or "navy blue." These colors are NOT to be confused with Indigo. Thus, aiiro CAN mean "blue," and in the case of Noel, it does.
Other Arguments
First, there's the common sense factor. I mean, just look at Noel. Her hair, dress, and mermaid tail are deep blue in color. Indigo, however, is an entirely different color - a mix of blue and purple, to be exact. And I don't think that "indigo" would be an accurate word to use to describe Noel's colors at all.
Also, aiiro is used to contrast with Hanon's shade of blue (mizuiro, or "aquamarine"). This is only necessary because we're looking at two different shades of blue. Indigo, however, is not blue. If Noel were meant to be Indigo Pearl Voice, Hanon would, in turn, probably have been just plain Blue Pearl Voice, since her particular shade of blue wouldn't need to contrast with Noel's. Er, does that make sense? Sorry for the convoluted explanation.
Finally, here's the Japanese wikipedia entry for aiiro. Does that look like indigo to you? I would definitely call that color "blue." And for the curious, here's a google image search for the color aiiro. You can see that a whole lot of different colors show up, some of which are definitely blue, some of which are definitely indigo. Like I said, the Japanese term aiiro can be applied to colors that English speakers would label as blue or indigo. So we have to go by what we see, not what the dictionary tells us.
Addendum June 2007
Well, Del Rey decided to translate Noel's color as indigo in the official English version of the manga. I disagree, of course, in my review of Del Rey's volume 05. But really, I've already laid out any arguments that I can make about the matter. So it's up to y'all to choose which color term you prefer.