Uncooked peeled potatoes will start to discolor quickly due to exposure to air.
To prevent this, place cut or sliced potatoes in a bowl of cool water until ready to use (up to 2 hours) to keep the nutrients in the potato.
A few notes:
It won’t affect the quality of the potato, but many find it unappealing.
Refrigerating cut potatoes uncovered will not stop this process.