Some minor yellow foliage in the Summer may indicate watering or heat issues. But it is usually not a big deal if you are talking about just a few leaves. Spots may or may not be a problem. A picture may help identify them but new hydrangeas should be ok with a few imperfections. Try watering the soil and never the leaves to keep spots in check. If these imperfections get a lot worse by the end of summer, post again.

You can try Holy-tone but I just use organic compost or composted manure. I already buy them anyways for other plants so, it is convenient to use them and they respond get well to composted manure. Cottonseed meal works well too.

But you just bought the shrubs; if they already came with those round fertilizer pellets, that would be enough for this year. Hydrangeas do not need as much ferts as plants like roses do. A single applicant in most places will be fine, assuming you do not have minerals deficiencies.

Too much of fertilizers can also raise nitrogen levels in the soil and that reduces blooms so try not to over do it. Unless your soil is deficient in some minerals, you could even skip fertilizing in some years and the shrubs/leaves/blooms would be fine with minerals supplied by the decomposing mulch.

In the northern half of the country, you could just apply those general purpose, slow-release fertilizers once and they would basically last through the growing season. Unless you are located way south in the southern states, do not use fertilizers after June-July to make sure that the plants will go dormant at the proper time. Additional applications of nitrogen-light or zero nitrogen fertilizers may be ok there in late Summer or early Fall. But I am talking places where dormancy occurs in December or later.

As Luis said, a new plant shouldn't need fertilizer, and you can often do more harm than good with fertilizers, such as what Luis cited about excess nitrogen causing more leaves and fewer flowers, but also excess fertilizer can wash out of soil and into nearby waterways. So unless a soil test tells you something is missing I wouldn't add fertilizer now. If you want to add a slow release product like composted manure next spring that shouldn't be an issue, but I honestly never add fertilizer to mine and it grows very enthusiastically.

What you are likely seeing is transplant shock. The plant has had its roots disturbed, the soil and sunlight levels are different, and the watering regimen has changed, so it is reacting to all those changes by looking a bit stressed.

The best thing you can do for this plant is to keep the soil evenly moist (not soggy or dry). Stick your finger in a few inches to check, both in the original rootball and in the surrounding soil. Water with a long slow trickle with a hose or a jug with pinholes punched into it so the water soaks in deeply. I cover the soil under the plant (but not actually against the stems) with a couple of inches of mulch, either shredded wood or compost, which will over time break down and provide your plants the nutrients they need to grow. This will help keep soil moisture even and help reduce weeds.

You don't say where you are, but in most of the country Annabelle appreciate shade in the hottest part of the day. Mine here in central NH gets sun from early morning until early afternoon and then is shaded by a building and it grows happily with little attention from me after its first couple of years. I do need to remove suckers to keep it reasonable sized but it will be a while before you have to worry abou that. If you are in a warmer, more southern region, like Luis in TX, it will need more shade to thrive.

If you see leaves looking limp, give it some temporary additional shade for a few weeks while it settles in by placing something like a lawn chair where its shadow will give some shade while the plant adjusts to its new home.