COLORS is a Vocaloid fan book that came out during the Vocaloid Master 9 (ボｰマス９) from the circle Ritornello, whose main artist is Mitosa (みとさ). You can check out her (or his, I never know this kind of thing ^^;) art work on Pixiv as well.

The Voc@loid M@ster 9 event was just a little while after Comiket (on 09/06/2009) so I was surprised to see that there were so many new illustration books, that hadn't come out during C76. But I guess when you're a Vocaloid fan yourself (as the artist seems to be) you'd prefer premiering at the Vocaloid only even!

There's definitely a bias towards Kagamine Len and Rin in Mitosa's COLORS. Out of the 15 illustrations, 6 of the illustrations feature just Rin or Len or the two of them together. And since a lot of the other drawings are group shots, Rin and Len are in nearly every picture in the book.

Hatsune Miku does have a few entirely on her own though. A couple of the illustrations were done for music videos on Niconico Douga. There is a particularly nice one of Hatsune Miku stepping out of a liquid screen and onto a music keyboard ^^

And now for one with everyone in Santa Claus suits! Though not pictured, there's also a full page collage of Megurine Luka. I like the newer Vocaloids, but they don't appear as often in doujinshi as the older ones.

Speaking of newer ones, there are also a couple of pictures featuring Gumi, though one of them is a little silly (see above). The other, with Gakupo and Gumi is a guest artist illustration by Kazune. Kazune also has a really colorful style, similar to Mitosa's and also worth checking out. Though it's not a collection I could see myself going out of my way to get, it's definitely nice enough to bundle up with other doujinshi if you're doing a big shop! ^^