There are some hydrangeas that produce mixed color blooms (blues, purples, pinks) naturally. One of the most advertised ones is Bloomstruck from the Endless Summer Collection. I am not sure if acidifying one of those guys will produce all blues at some point or not. You may need to get the soil pH into the mid to upper 5's. Just a comment. Let me know if continued acidification down to the 5's helps should you ever try that. I am just curious because I asked the Endless Summer folks about Bloomstruck and they replied that in higly alkaline or highly acidic soils, it may not have as many mixed colors. When I pressed for more details, I got silence so maybe they do not want to "spill the beans".
I do not have one like Bloomstruck; mine are usually just one color (white or pink). My 7.6 acidic soil is typically amended in Spring to prevent signs of chlorosis. Hydrangeas in good composted soils tolerate the alkalinity with this but, should they 'email me' about having chlorosis ;o)) in early Fall, I will take a look and amend the soil again.
There is only one unnamed lacecap rebloomer that gets lots of amendments (aluminum sulfate) though. It is a colored hydrangea that is part of "an experiment".... meaning, I am trying to produce purple flowers. Purples are hard to produce because they require a narrow soil pH range and I try to get the soil pH from slightly acidic to neutral. And of course, I usually get purples by amending the soil. Some years, I get a weird purple that looks great in the late afternoon hours. But eh, this is nothing I could replicate well, mind you. This is as experimental as the recipes that I use when I cook. A pinch of this, and so forth. Hee, hee, hee.
What types of hydrangeas are the ones that are yellowing and how much? Do you have pictures of those? Take one of the shrub so I can get a feel of how widespread is the yellowing, then close ups of some yellow leaves. Summer is here almost so this could be sort-of ok.
Were you having pests that needed taking care of in May and again in May? Or was this done as a preventative due to insect problems on prior years around those dates?