Answer: It sounds as though your Acer negundo 'Flamingo' ("Flamingo" Ash-leafed maple) is undergoing some severe stress. Unfortunately, based on your description, I am not able to make a diagnosis long distance. Yellowing could be caused by transplant stress, over or underwatering, a poorly drained location, heat stress, overfertilizing, accidental chemical exposure such as herbicide, planting too deeply or other planting technique issue, sudden cold stress, riding home in an open vehicle, or even an insect problem -- or a combination of factors.
Since it is a new plant, I would strongly suggest you consult with your retailer, especially if it is under warranty. You might also wish to consult with your county extension. They will be familiar with the recent weather extremes and their potential effects on newly planted trees. I'm sorry I can't be more specific for you.
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