While those are all good reasons (with some kernels of truth to each), the primary reason is actually this: I wanted each book in the series to represent a color on the spectrum as described by the famous 17th century scientist, Isaac Newton.
|Newton's Color Wheel|
I also liked the fact that Newton's use of the word "spectrum" originated from his work with light waves. In Spectraland, waves of light, sound and other forces make up what is known as the Aura, a field of energy that Joel and others with his ability are able to manipulate through the use of musical instruments called wavebows.
Also, in the first chapter of Songshell, there's a Newton reference that I wrote before deciding on the whole seven-book series thing, where Joel - a huge Isaac Newton fan** - talks about Newton's theory of air resistance (which turns out to be a pretty significant plot point later on). Pretty cool coincidence, I must say.
And finally, I had always envisioned the overall tone of the series getting darker as it went along, and that tied in perfectly with how the spectrum of colors eventually progresses to indigo and violet. So, Secret of the Songshell is the "Red Book." Book Two will be the "Orange Book," Book Three will be the "Yellow Book," and so on. Remember the rainbow color acronym ROYGBIV? Yeah, it's like that.
Anyway, now you know why I'm planning to write seven of these things. And I can't change my mind even if I wanted to, because who ever heard of a rainbow with less than seven colors?
* Originally, I toyed around with the idea of subtitling Songshell "Book D" (since Newton assigned the music note "D" to the color red) and so forth, but I figured that doing so might be getting a little too esoteric. Who knows, maybe I'll revisit that idea someday.
** According to Wikipedia, the Cambridge psychologist Simon Baron-Cohen considers it "fairly certain" that Newton had Asperger's Syndrome.