Answer: Purple tint can indicate a nutrient disorder or can be the result of extreme weather (a sudden cold snap, for instance). Lilac's prefer a slightly alkaline soil so if your soil is too acidic, necessary elements can be bound up in the soil and unavailable to the plant's roots. Typically, a phosphorous deficiency can cause the purple tint. Have your soil pH checked to see what you're working with, and then feed with a fertilizer high in phosphorous (5-10-5, for example).
Best wishes with your lilac.
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