Both Knuckles and Amy have been confirmed to ‘appear’ in Sonic Colors DS. This doesn’t mean they will be playable, they will just come in for no reason and add nothing to the story, like they usually do.
“The DS version introduces the other characters like Knuckles and Amy as kind of the guest characters. They don’t have any direct influence on the gameplay but they act as the narrative to the background of the story, so if you play the DS version you get a better understanding of the story for Sonic Colours.” –Takashi Iizuka
This is the part where I complain that Amy should be in the Wii version, so we can get better shots of her panties. But if I said this, you guys would all think I’m a weirdo, so I will not.