Donna, where on the plant are the leaves? Lower leaves turning yellow could be a nitrogen deficiency - which doesn't usually happen in soil of average fertility whether you fertilize or not. And it's a 'not' here, I top dress with compost every year then don't do anything else for hydrangeas other than water when needed.

If top leaves are turning light green or yellowish with dark green veins its usually chlorosis, low iron uptake caused by soil with too high PH.

So, where are you seeing the lighter leaves?