To be honest I think Wistera is the suspect, but I do not want to copy PoisonusShroom. So I say Azure. Why? He might have put chemical stuff in the food since he has a higher knowledge of substances than us.
Azure might have injected a posion that was harmful to the cherry tomatoes and made them moldy and then sliced some of them and leaked all of the juices into Grape' salad. Which the posion in the juices and cherry tomatoes thefore killing Grape.
Well Good luck to everyone alive. I knew this would happen... If you luck carefully in the way Magenta was killed you can find a he which means that the only killers are Rainbow, Azure, and Orange.
Hmm. I believe the murder was caused because when Vermillion went into the garden room and when Skyan pressed those buttons it must have activated something and the bomb was perfectly timed. So that means that Skyan did not do it. So I say Turquoise.