Hello, tinamcg. I do not try to change hydrangea color but this is my understanding of the process.

Before adding aluminum sulfate, I would make a soil test. A soil test is necessary to tell you if you already have "enough" naturally occuring aluminum in the soil. If you do not then apply aluminum sulfate to increase aluminum levels and lower the Ph. Aluminum sulfate should work fast since its application causes a chemical reaction.

Once you measure "normal" levels of aluminum in the soil or if you already had "normal" levels of aluminum in your soil, you may want to apply elemental sulphur (a.k.a. flowers of sulphur) in order to further tweak the PH Level.

Contrary to aluminum sulfate, Elemental Sulphur takes several weeks to acidify the soil because it first has to be oxidized by bacteria (here you have a biological reaction, not a chemical reaction).

I would add chemicals monthly but only thru the growing season in Spring and quit when summer starts. Not particular reason for that though. I do not want to introduce 'problems' with the addition of chemicals at a time when the plants experience high temperatures, lack of humidity and drying winds.

Regarding your desire to control the color to lilac or blue, be aware that color change is a project that takes several years and requires lots of applications and soil tests (to see how things are progressing). Also, it is very difficult to the intensity of a color (how strong or pale the color is) so you may need to experiment with other varieties if ES just does not want to attain the type of color that you want.

Luis