Answer: It isn't unusual for odd colored leaves to appear in plants from time to time but I don't think the entire fern will be affected by a few odd fronds. If it were a grafted tree or shrub the odd color might be considered a sport, but it's usually due to cultural conditions when an herbaceous perennial produces off-colored leaves. Since the leaves will be gone in a few weeks, I'd leave the plant alone this year. Next year the fronds should be of the expected color. If not, it might be that some spores have germinated and begun new plants within the original plant. If so, it will be time to dig the plant and divide it, discarding the roots associated with the off color foliage.
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