Potatoes which have been exposed to sun turn green, the colour is caused by chlorophyll, but it indicates that there will also be increased amounts of solanine in the whole potato. It does not help to cut away the green parts, and solanine is not removed by boiling (but deep frying does). You have to eat quite a lot of green potatoes to be poisoned, though, it is said that there have been no fatalities in Europe the last 50 years.