" what can I do to affect the end of season color? "

Not much, really. While the chemical composition of the soil will determine in-season flower color, the coloration they develop at the end of the season is more a factor of siting and climate. Not too much sun, not too much heat, adequate irrigation, etc.

What you can do is attempt to alter the in-season flower color to a uniformly lightish blue - no purple or pink. IME, the paler blue flowers almost always age to a light teal blue-green that is close to iridescent.

Wonderful insight and advice! Thank you so much!

I agree with Gardengal- I have fairly neutral soil so most of my ES hydrangeas are a mix of light pink/blue that age to a light green. I’ll attach a photo!

These are beautiful!! I hope I can achieve the same this year!!! Mine ended up this color last year but I didn’t try amending the PH. Fingers crossed!

You can control the initial bloom color of a mophead hydrangea by soil pH control as long as the variety does not produce white blooms but after 1-2 months, they will begin a series of color changes that are less related to soil pH and more to the variety being grown, nutrients, weather factors, etc. Those are difficult to control factors. Be aware that if you do not happen to like a certain color during this progression of colors, you can deadhead them bloom by cutting it above the first pair of leaves without affecting future blooms in spring. And no, you will typically not get a second bloom in the same growing season from the stem if you deadhead; but you will normally get a bloom next spring. I would ask for a few cuttings when you see any hydrangea that produces those shades of color and then see if/how ell they replicate in your different soil nutrients, sun exposure, weather, etc.