First, from a botanical standpoint, it is crucial to understand what you are looking at. The iconic white part of a Peace Lily (Spathiphyllum spp.) is not a true flower petal. It is a specialized leaf called a spathe. Its sole evolutionary purpose is to attract pollinators. The actual flowers are the tiny, rough-looking structures clustered on the central spike, known as the spadix. The brilliant white color of the spathe is a highly effective visual signal to insects. However, this whiteness is not a permanent feature. It is a temporary, high-energy state for the plant.
When your Peace Lily's white spathe begins to turn green, this is most often a completely natural and healthy process. As the spathe ages after its initial display, its function shifts. The plant no longer needs to expend energy on maintaining a bright white "beacon" for pollinators. The green color you see is chlorophyll. As the spathe matures, it begins to produce chlorophyll, just like a regular leaf. This allows it to contribute to the plant's energy production through photosynthesis. Essentially, the spathe is being repurposed from a showy advertisement into a productive, energy-creating organ. This is a sign of a smart, efficient plant, not an unhealthy one. If the green color is undesirable for aesthetic reasons, you can simply snip off the aging spathe at its base to redirect the plant's energy to new growth.
If the spathe emerges green or develops a pale green hue very quickly, the plant is likely responding to its light environment. Peace Lilies are adapted to the dappled light of forest floors. When placed in an area with very low light, the plant may produce green spathes. The biological reasoning is sound: in low light, the plant prioritizes photosynthesis over reproduction. A green spathe can contribute to energy production from the very start, which is more valuable to a light-deprived plant than a white, non-photosynthetic structure. Conversely, excessive direct, bright sunlight can scorch the spathe, causing it to turn brown prematurely or develop crispy, brown tips. The plant is essentially suffering from sunburn, which damages its cells and chlorophyll, leading to the brown, dead tissue.
Brown coloration on the spathe is almost always an indicator of stress or the end of the life cycle. A spathe will naturally turn brown and wither after several weeks or months, which is the plant's way of concluding that reproductive effort. You should then prune it. However, premature browning points to an environmental problem. The most common cause is an issue with water. Underwatering causes the plant to dehydrate. The spathe, being a large and relatively thin structure, is highly susceptible to water loss. It will turn brown and crispy as its tissues die. Overwatering, which leads to root rot, is equally damaging. A rotting root system cannot absorb water or nutrients, effectively starving the aerial parts of the plant. This also results in browning, often accompanied by a general droopiness or yellowing of the leaves. Low humidity can also contribute to brown tips, as the plant loses moisture to dry air faster than it can be replaced by the roots.
While less common than light or water issues, nutrient levels can influence spathe color and health. A significant lack of essential nutrients might result in weak, small, or off-colored spathes. More frequently, an excess of nitrogen can encourage the plant to produce lush, green foliage at the expense of flowering. The plant interprets high nitrogen as a signal to grow vegetatively (leaves and stems) rather than reproductively (spathes and spadixes). This can lead to fewer flowers, or flowers that are greener. Furthermore, a buildup of salts and minerals from fertilizers or tap water can "burn" the roots, impairing their function and leading to brown tips on both spathes and leaves, similar to the effects of overwatering.
