The lack of fertilizer is probably not the cause of this yellowing. Your peony is probably beginning to go dormant with the heat and lack of rain we are experiencing now. If you haven't been watering it regularly during this dry spell, you should give it a good soaking now.
I would wait and fertilize it with Plant-tone in the fall around October. Once the peony foliage becomes completely yellow, it can be cut back to the ground. They will come back beautifully next spring.