purple is a bit of a challenge. they are super cool looking, although you have to kill it with them. it also doesn't take much light to wash them out. what type of speaker grills are you trying to light up? the type of grill lends itself to different led light options.
i would not recommend pulling power from your speaker terminals or speaker wire for illumination. most likely, you'll end up with noise coming through your speakers.
as the led's draw such little power, you can easily get away with 20 gage