The most common reason for yellowing of geranium leaves is over-watering. Cool temperatures can also cause geranium leaves to turn yellow.
Geraniums are best suited for warm climates; exposure to cold temperatures in the air or water can cause the leaves to turn brown. Cold, wet weather can do the most damage to geraniums. If the leaves are yellow with light-colored spots, the cause may be excessive watering. Geraniums are drought tolerant and can be damaged by too much water.
A nutrient deficiency can also cause yellowing in geranium leaves. Using a water-soluble fertilizer containing nutrients can alleviate the problem. Yellowing could also be a sign of plant disease such as a fungal infection from verticillium.