Since hydrangeas are acid-loving woody plants, I would first check the soil pH with one of those cheap kits that give you some kind of visual (a color or a number) result when the pH number is low (which means the soil is acidic and has a pH number of less than 7.0) or high (which means the soil is alkaline and has a pH number above 7.0).. When your soil pH gets high (alkaline), some hydrangea leaves tend to turn a lighter shade of green. On a kit that gives you numerical results, I would try to get the soil pH below 7.0 although many leaves will be ok sitting on neutral soil (7.0).

Potted hydrangeas suffer a little more from alkaline if your soil is alkaline so some people with this problem use rain water instead. To acidify the soil, you can use greensand, aluminum sulfate, garden Sulphur or liquid iron-chelated compounds sold at most local nurseries.

Another thing to keep an eye on is powdery mildew, common in mopheads. Sometimes, when the infestation begins, the leaves may turn a lighter color too but eventually, the leaves will develop gray or white spots.. Water the soil instead of the leaves to help minimize this problem. Some people use a 20% milk & 80% water solution, sprayed on both sides of all leaves if they detect PM on their hydrangeas.

Good luck,
Luis