I believe my hydrangea has powdery mildew which was present when I bought it but got worse. I don't overhead water. I sprayed with a baking soda mix yesterday and think it helped but noticed how bad the plant looks even after this. A couple of months ago I used (I think) a Seasol fertiliser on it and noticed the sides of the leaves browning. I haven't used it since, but the browning is getting worse. I am watering less now that the weather is colder (in Melbourne). The leaves have what I would describe as a light green and yellow mottled appearance. Thanks.
Hydrangeas are deciduous, so the discoloration is likely down to natural processes that take place as the leaves are about to be discarded.
Otherwise, Hydrangeas,depending on variety, are large plants - you don't say how long you've had the plant, but it won't do terribly well contained in a pot for very long,though they will do longer if you pot on into larger containers as time goes by. Still better planted in the ground though I'm afraid.