Place the grapes in a bowl and cover with water. Let sit for a minute or two.
While the grapes rest, place the Kool-Aid powder (for sour candy) or sugar (for sweet candy) in a gallon size Ziploc bag.
Drain the grapes and shake as much excess water off as possible without dabbing them with a paper towel.
Dump the grapes into the bag with the powder or sugar. Seal the bag and shake all around until the grapes are coated.
Remove the grapes with tongs or a toothpick, or dump them into a dry colander to remove them from the coating.
Transfer to a plate or container and store in the fridge for an hour, or until dry.
Notes
Use other flavors - I liked using the red flavors as it made the color pop up better for photographs. However, you can use any Kool Aid flavor or off-brand powder you like.
Go with lighter coverage - If you don’t want such a thick layer of flavoring on your grapes, you can opt for dipping them one by one. Use a toothpick and dip them into the powder to control how much coverage they retain.