Hot on the heels of Pop'n Portable comes a Pop'n spinoff for the DS. Amazingly, in some ways it's even less intuitive than its wackily-controlling PSP counterpart, partly due to all the note types.
Red: Hit the note.
Yellow: Hit the note and slide up.
Blue: Hold the note and let go right at the end.
Green: Hold the note while wiggling the stylus a bit, let go at the end.
(Ultimately, the yellow ones are the worst idea of the lot, because they're weird to time and always come off sounding strange.)
The timing seems less forgiving than usual, and poking around with one stylus is a little tricky, but it's still a fun game, more or less. I think Pop'n Portable beats it, but if you don't have a PSP, this is worth checking out.