Why are my potatoes a grey color? After I peel the potato, little grey lines appear throughout the flesh. Are they still good to use?

This sounds like it is a chilling injury, which will show grey lines of discoloration. If the lines are very distinct and brown in color, then the potato may have zebra chip. Either one will be okay to mash and eat. However, both will oxidize quickly and cut and boil soon. Don’t cut the potatoes, let them sit, and then boil them. The color may become somewhat off. Also, both potatoes will have higher sugar content, so the potato’s taste may be on the sweeter side.