I make R/W&B Jello cups for the 4th and other such holidays.
You'll need red and blue jello, cool whip and clear plastic cups.
First, make the red and blue jello. You might need to make a lot, depending on how many cups you're making and how big they are. When it's firm, alternate layers in the cup with a colored jello on the bottom, then cool whip, then a different jello and top with cool whip. Everyone likes jello!
Cupcakes with white frosting and red & blue sprinkles on top. If you really want to get funky, use a plain white cake mix for the cupcakes, but add a little Koolaid mix to the batter. It will be vibrant red or blue (depending on the flavor you use). (make sure to use the unsweetened kind)
Fruit salad with strawberries,blueberries and honeydew melon. I know honeydew is really green, but it's so pale it will look almost white (unless you have the color police at your party, then you'd better skip it!)
Cake with the american flag made out of strawberries and blueberries on cool whip frosting.
Make lemonade and have ice cubes pre-made out of red and blue koolaid. Or make the ice cubes with water but put a slice of the fruit in each block of ice before it freezes.