Potato with purple veins!

Today while cooking with potatoes when I cut open a potato(raw) which was looking perfect from the outside showed purple veins. I threw it in my kitchen garden but than I wondered what it could be

Questions

1. Why did my potato developed these purple patterns?
2. What could be that purple pigment?
3. Are such potatoes Edible?

PS: The potato was not old. It was quite fresh.