How much food coloring would it take to turn 1000 gallons of water dark blue?

Hard to answer correctly, as food colouring comes in various concentrations, but I reckon you could probably do a good job with about 1:100 concentration. (10 gallons per 1000)

Test it by using 10 drops of water and 1 drop of food colouring in a small tube, and adjust as desired.

Roughly 10 small bottles of red food colouring should do the trick for you.The red colouring is a very concentrated colour.Once added if it is not dark enough add a touch more.I wish you all the best for the 4th july.My son and daughter both share their birthdays on this day and they are not twins.