Wow, that's pretty. That size would work well where the plant is now.
Do you have it in full sun? I'm in zone5, but I wonder if mine is getting too much light (because of the older leaves seeming to bleach (not burn though).

Chris,Z6a it get about 6hrs Sun, but no fertilizer

Since it is a mophead that ONLY produces invisible flower buds in July (where I am... in the South) to August (where you live), it may be a good idea (a) not to do any kind of pruning after the end of June (to promote blooms) and (b) use winter protection once the plant goes dormant (take off the winter protection a few weeks after your avge date of last frost in Spring). since the invisible flower buds probably already formed for Spring 2018 and are located at the ends of the stems, I recommend not pruning any more (in case you were planning to do any pruning), also, do forum finds/searches for "winter protection", which involves putting chicken wire around the shrub and filling it with mulch or dried out leaves to protect the ends of the stems (that is where the invisible flower buds are located). These buds either do not reliably survive your Zone 6 winters or may be getting pruned off sooo, winter protection & not pruning will improve the odds of getting blooms again.

Too much sun makes the leaves in direct contact with the sun turn
yellowish/whiteish (the leaf veins are affected too). However, the leaves
underneath -since they are protected from the sun by the topmost leaves-
remain dark green.

If it is getting sun bleached then it is getting too much summer sun. Normally, a good location for a mophead is a place where it gets only morning sun, gets dappled sun or gets full shade with bright shade. Obviously, making that change would require transplanting it once it goes dormant (minimizes transplant shock). Other alternatives would be to build some structure that provides shade or use 35% shade cloth. The shade is actually only needed around the summer months as this is the time when the sun is "strongest".

On the other hand, if you notice that the leaves are turning light green to yellow but the leaf veins remain dark green then that is caused by the plant not being able to absorb iron, a condition called iron chlorosis. Depending on what stage this issue is in, you may have a several random leaves throughout in this condition or almost all leaves if the condition has not been corrected after many weeks/months.

Iron chlorosis is very common where I live for example and is caused by the soil being alkaline (meaning soil pH is above 7.0) . To address that, I amend the soil with either greensand, garden Sulfur or aluminum sulfate, added per label directions year around (if you stop for long periods of time, that will make the soil go back to its normal alkaline state). You can also use iron-chelated liquid compounds (sold in plant nurseries) as these resolve the problem the problem faster than the other products (but these must be reapplied more often). Still, both types of products do not work overnight so be prepared to wait. And wait. Just keep reapplying on the schedule required by the product (but probably stop when your soil freezes in winter).

Wow, thank you Louis, this is super information.
My aim is for low-maintenance gardening, so this sort of soil amendment is out. However, I do have a perfect morning-sun spot in a new bed I just built along the east side of the house. There is a lot of compost there, so the flowers it may eventually produce will probably not be blue, but that's fine, too.
It's a bit crowded where it is, anyway, so I'll move it next month.

I have a couple from the supermarket. You will want to offer winter protection. They need it. Mine are proving to be rather hardy but refuse to bloom without some winter protection. Mine won't get very big I think, but I'm ok with that.