Well, it's a little tricky but here goes.....

A pink/blue mophead such as your ES & Nikkos will turn purple because purple is the color in between pink (high ph) and blue (low ph) but is extremely hard to maintain unless your soil naturally has the perfect ph but it will never be a deep dark purple because those varieties are not deeply pigmented. If you are adding AS to make them blue, I am assumming that you don't have acidic soil naturally..??

Now, there are certain cultivars that are heavily pigmented to the pink side. Some are even called 'red' varieties. These cultivars resist going blue in acid soils. Therefore, when planted in acid soils, the closest they will come to blue, is purple and because they are heavily pigmented, they can be deep purple or wine colored. A few cultivars that are famous for deep purple/wine colors in acid soil are; Merritts Supreme, Mathilda Gutges (aka Royal Purple), Glowing Embers, Ami Pasqiuer, Bottstein, Gertrude Glahn, Leuchtfeuer(aka Firelight), Masja, Paris, Red Star, Westfalen. I'm sure there are others, but the easiest thing to do to be sure that you get what you saw & want, is to go back to where you saw them, knock on the door and ask the homeowner which cultivar he/she has. Chances are, they will be flattered and very accomodating. Maybe even share a cutting!! Most gardeners are funny that way....yg

Yes, as per YG, my Glowing Embers has always been a nice purply color. It has never been any other color, so the area must be acidic(next to my foundation). It's a nice sturdy plant too, holding up well in these torrential downpours we've been having. I was driving around my mother's neighborhood today and saw tons of blooming pinks/blues, but it sure is a nice change to come home and see my "purple" Glowing Embers!

Yellowgirl,
I would just like to thank you for taking the time to answer that question, I have been pondering that one myself. Good explanation :) I had the ph thing down -just not the right cultivar for a divine purple that I have been coveting ;P
On another color subject :The other day I saw one that was dark dark blue almost a blue black color. Much darker blue than usual dark blue and much darker than I've ever gotten any of mine and not purple at all. I wonder how they managed that? Perhaps I will pay a gardenly visit lol.
~SJN

Update: Per your suggestion, I did go by and talk to the homeowner with the beautiful purple hydrangeas. Unfortunately the gentleman who lived there told me he didn't have a clue what kind of hydrangeas those were as they had been there already when he moved in. He was nice enough to offer me a cutting and I am attempting to get it to root (my first time for taking a cutting).

Thanks for the wonderful advice above and for 'naming names.' I will print this information out and use it as a reference when I shop for my next hydrangea as I MUST have a purple one since my garden has a purple/blue color theme. I also need to test my soil to get an exact pH level but I do know that my hydrangeas are pink unless I use AS in the soil around them.

One last note; I've heard that some gardeners use other additives (bone meal ? fish meal?) to get more intense colors for their hydrangeas. Does anyone know if there is anything to that?

Thanks