I would discuss this a florist and have them take care of the arrangements. Yes, it will cost more than if you do it but this sounds stressful and not something that you want to messing around as the date gets near. I can just picture myself checking the plants daily to see how they are doing! Macrophyllas tend to start blooming earlier, around May over here so you could definitely have some white and blue blooms provided by them by July 31st. The green ones could be obtained from Annabelle or Limelight Hydrangeas or they could be spray painted from white macrophylla blooms. Too much work to do yourself IMHO. Problems to consider: droughts, wind damage, dog damage, bloom colors fade, etc. Can it be done? yes but I agree, you need a Plan B in case something happens.