I had purple fringe problems when I first used my 20D at night with a 70-200 L lens, and man was I pissed. It was pretty bad. Well, I found out waht it was. I accidentally bought glass, no couting UV filters--that I use on all my lenses. after I replaced them with Multicoated filters, the fringe went away. BTW, using the Multicoated Pro Hoya UV filters seems to cut down even more fringe than using the bare lens. However, make sure you get MULTICOATED, not "coated" etc.
Same problem happens with multiple lenses.
All so far...Canon Ef and EFS
Seems to be a weird color break that shows up as purple in transition areas.....samples attached.
Shows up in color and in images converted to RGB B&W.
Check these out.