Answer: Based on your description I am not certain if you have Coryllus avellana "Fusco rubra" or Coryllus maxima cv. purpurea. However in both cases the dark purple foliage will gradually turn green as the summer progresses. Using a different fertilizer would not affect the color and you certainly do not want to overfertilize it (and encourage excess growth) since it is in a container. As long as it seems healthy overall, I would not recommend any changes.
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