Just like apples or pears, potatoes have a tendency to discolour when sliced. They turn an unsightly shade of brown when they've been exposed to air for too long. This oxidisation process isn't harmful, but it can make the potatoes appear unappetizing. To keep your sliced potatoes looking clean and white rather than turning the colour of a dirty dishrag, there are simple ways to stop the oxidisation process.
Slice the potatoes and submerge them in a bowl of cold water.
Add 1/4 cup of lemon juice or vinegar per qt. of water. This will stop the oxidisation process.
Store the potatoes in a cool place until you are ready to cook them. However, do not store them inside of the refrigerator. The cold environment will convert the potato starches into sugar. They will become sweet and turn dark brown when cooked.
Add ice cubes to the water if necessary to keep it cool, but not too cold.
Drain the potatoes when you are ready to use them. Pat them dry and proceed with your recipe instructions.