Good morning Mary,

When the leaves start turning yellow but still have prominent green veins, this indicates an iron deficiency. This most commonly happens in alkaline or heavy clay soils. The 2^nd link is the link from the 1^st link that explains chlorosis & how to treat it.

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/hydrangea/leavesdiscolored.html

https://hort.extension.wisc.edu/articles/chlorosis/

I’d also recommend getting your soil tested to see where there may be nutritional deficiencies. You can request a home test here:

https://homesoiltest.msu.edu/get-started

Good luck & have a great day!