There are three reasons the leaves of a hydrangea plant would turn yellow: water issues, nitrogen deficiency and iron chlorosis. Water issues are the easiest to fix, but it is still possible to restore the leaves from the other two conditions as well.
Although hydrangeas do require a lot of water, it is still possible to overwater the plant. If the leaves start turning yellow, cut back on the watering to improve the condition. Nitrogen deficiency of a hydrangea is usually expressed by established leaves turning yellow, and it happens when the pH balance of the soil is too high. Add fertilizer with nitrogen, but not phosphorus, to fix the issue. Iron chlorosis occurs in new leaves because the level of iron in the soil is too low. To correct the deficiency, add iron supplements to the soil.