Some possibilities of leaf yellowing:
* A nitrogen deficiency can cause a complete yellowing of leaves but this is rare. You can correct this problem by adding a salt like ammonium sulfate or organic sources like blood meal. However, it may be more useful to investigate why there is this deficiency or if the deficiency is caused by a different problem. For example, the roots may not be able to absorb oxygen and other minerals if there is a lot of water in the soil. The excess water may be caused by overwatering or by excessive rainfall. To rule out too much water, you could get some soil in between two fingers and press to see if the soil produces water droplets. If it does, the soil is too wet. Inserting the two fingers to a depth of 4" or so.
* Nitrogen toxicity (too much nitrogen) can also cause some leaves to completely turn yellow. In this case, the reason is usually over-fertilizing. Or using fertilizers that contain a lot of nitrogen (the first number in the NPK Ratio). Or using that type of fertilizer too often. This toxicity will cause extremely dark leaves... with some leaves turning yellow. A soil test can tell you if you have too much nitrogen. Since hydrangeas do not need much fertilizer, I would use instead organic compost or composted manure with your sandy soil instead of the chemical fertilizer.
* A lack of minerals or difficulty absorbing minerals can cause yellowing of leaves that keeps leaves light green or yellowish while the leaf veins remain dark green. Lack of magnesium or iron for example, could cause this type of chlorosis. The soil may not have enough of these minerals (specially in sandy soils like yours) or the roots may be unable to absorb them because the minerals have formed chemical bonds with other minerals. The reason for chlorosis may be due to overwatering, lack of the minerals or due to alkaline soil that blocks the uptake of the minerals. A soil pH test can be used to rule out alkaline soil problems. Checking the soil for too much water can be done to rule out overwatering. My late aunt in Ocala, FL used to add organic compost when planting and also a 1/4 to 1/2" layer every Spring. A soil pH test kit can tell you if your soil is too alkaline or too acidic.
* Too much sunlight can also cause leaves to completely or partially yellow out. In such a scenario, only the leaves in direct contact with the sun would yellow out. The other protected leaves would remain dark green. Hydrangeas like Nikko Blue will do best in either morning sun only (say, until 11am or so), dappled sun or in full but bright shade. No afternoon and no evening sun.
* Winter damage can also cause leaf yellowing. If a plant breaks dormancy too early, the sap begins to flow. If a late freeze/frost hits when the frost is flowing, the leaves can turn a variety of colors: yellow, red, purple, dark green, etc. Picture 3 shows signs of this. Then you have to wait and see what the plant decides to do with the damaged leaf. Sometimes parts of the leaf brown out. Other times, they stay yellow and fall. New foliage may take 2-4 weeks to come out but it will be up to the plant to decide where to leaf out.
* Heat stress can also make some hydrangeas develop complete yellow leaves. For example, during the summer months, some hydrangeas will tend to lose leaves at the bottom of the plant. Some people call this "showing their feet". You could minimize the chances of this by always proving 2-4" of mulch (do they have any mulch?).
* Powdery mildew can also cause some leaves to yellow out too. But this fungal disease usually displays some white or grey splotches first. I could not detect signs of PM in the pictures.
* There are some cases where this is normal. There are hydrangeas whose leaves start yellow and stay yellow for a long time. One example is Sun Goddess. The more sun the leaves get, the faster they will change from yellow to dark green though.