I generally apply whatever amendment I have handy this time of the year and re-apply again in the Summer (if I remember or notice leaf problems). Since my purpose is not to change the color but rather to bring the soil pH down (my soil has adequate amounts of aluminum), I tend to apply soil Sulphur most of the time. So I bring the soil pH down but not that much; it is still alkaline but the hydrangeas' leaves tolerate it and stay green; they only show signs of iron chlorosis by the end of Summer and only on some years. I have also used green sand, aluminum sulfate and iron sulfate as amendments.

You best start by determining what is your soil pH. And then proceed to lower the levels if they are too high. A soil test will be very useful. A soil pH kit may help some but they do not give you numeric readings so it is difficult to compare the results from one month to another month if you cannot remember how green or how pink the color was a month ago. If you can find a kit that gives numeric readings, although more expensive, it may help you control things better.

You will have to continue amending the soil regularly. If you stop, the soil pH will go back to normal. So be ready to amend every month or two/three months per soil test recommendations. The soil test analysis should include recomendations on what to apply to lower the pH. When you request it, please indicate that you have hydrangeas and would like to know how often to amend the soil in order to get the soil pH to whatever level you want (say 7.0 which is neutral).