I know that you think that this is an original idea--something different--, but it isn't. Many many people have tried this thinking the same thing you did. But if you understand the biology of the plant and how it "eats", you will understand that adding fruit juice to your plants will do nothing and actually be detrimental. In addition there is a good chance that it will attract bugs. I also am not sure you understand the concept of flushing--flushing, if done at all, is done with pure water to wash excess nutrients away. But using fruit juice will leave you with a sticky mess. The plant is not going to uptake any of this fruit juice into its buds and affect the flavor--that is just not how any of that works. While red dye may add a bit of "color", understand that at some later date that you will be taking that red dye into your lungs when you smoke it. If you want purple plants, it is best to buy purple strains. I am not even a big fan of lowering your temps below 60 to get purple. Temps below 60 are detrimental to good cannabis growth. They pretty much quit growing when temps get below 60, so that seems a bad trade-off for some artificial color. There are really no upsides to either of these things. You will be far better off to simply take care of your plant the best you can with (scientific) tried and true methods.