changing Hydrangea color

Hi all, my neighbor has a hydrangea this now blooms pink. She wants it to be "purple" again. What does she need to use to change the soil acidity?

Here ya' go, Dingbloom: http://www.gardenpower.com/projects/joes/99036.htm I'd quote exactly which one it is, sulfur or aluminum sulfate, that you need to add to acidic soil or alkaline, but I always get it backwards, so I'll just let that link speak for itself. Good luck!

Thanks Wingnut! that is exactly what I was looking for! I'll print it out for my neighbor. Thanks again!

Anytime! :)