I assume you mean the color of your peace lily (spathiphyllum) leaves are a duller green and not turning yellow. If your plant is root-bound, it may not be getter enough nutrients or water to sustain healthy leaves. Plants exist for only one purpose which is to reproduce. Since the bloom is the way they reproduce, the plant is diverting all of the nutrients and water to the bloom and away from the leaves. If your plant isn’t root-bound you may need to increase the fertilizer and/or water for the plant. As the plant grows water and fertilizer requirements may change. Increasewater and fertilizer by small increments, so that you don’t over due it.