Echinacea or coneflowers will turn purple due to stress. The number one reason they get stressed is too much heat and dry soil. Other stresses can be too much shade or poor drainage. You may live in area that experienced high heat this summer. Next year try mulching around the roots to keep the plant cooler and water when the top one inch of the soil is dry. I also like the rich red color  of "Tomato Soup" Echinacea. Looking forward to a cooler summer and redder flowers in the future. Keep growing, Marianne Binetti