Hi Laurel - here is a recipe that was posted here on the Recipe Link as tried-and-true by Shirley/AZ in 1998. I am not sure why they add both blue and green food coloring, but you can omit the green. Lemonade and 7-UP sounds good to me. I hope it helps.
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Blue Punch

1 gallon water
1/2 c, sugar
11(6-oz) cans frozen lemonade

Mix well and add:
1 T. blue food coloring
1/4 T. green food coloring

When ready to serve add:
11 (12 oz) bottles 7-UP and
1/2 ga. pineapple sherbert.
This serves 50-75 people.