I thought of the obvious, fruit salad, yellow cantaloup or mango, red watermelon, strawberries or raspberries, blueberries, kiwis, oranges or clementine. and purple dyed mini marshmallows

or purple grapes.
With Nancy's idea you can also use purple potatoes or cauliflower, even red onions are often more purple than red. There are always blue corn chips. I wonder if a few blueberries would turn water blue to dye potatoes?