Potato Plant Leaves Turning Purple?

I might have planted earlier than I should, we've just gotten 2 weeks of rain and cold temps. My plant's leaves have just come up, but are turning purple colored. It's not mold, is this a stress reaction to the cold and wet weather and what can I do to make them stronger? Also will putting manure down raise the pH too much? How can you tell if the pH is too high and what can I do to balance this out?

By Alex