The leaves of the dwarf Hydrangeas may turn red if there is a deficiency in in the soil. It is important to have the correct nutrients when growing hydrangeas because it is crucial to helping the proper growth of the plants. It is possible that a nutrient deficiency might cause reddish coloration to the leaves.

You may wish to submit some soil for testing from the spot in which your Hydrangeas are planted. Soil tests are available online at https://shop.msu.edu/products/soil-test-mailer?_pos=1&_sid=74ef9fc82&_ss=r; currently, home soil tests are $26.

Other reasons leaves may turn red. . .
---Too much sun: Hydrangeas like some shade, too much sun can burn their leaves red.
---Not enough water: if the soil is dray, their eaves may redden as a warning.
---Soil problems: the plant needs rich, moist soil that is not wet or dry. Acidic soil is an issue and makes leaves redden also.

Solutions for Hydrangea Leaves Turning Red

Prune the plant: The easiest way to recover from the reddish coloration of the leaves is to prune your Hydrangea plant. Pruning helps stimulates new growth and allows the replacement of damaged leaves. Ensure you know How to Trim a Hydrangea correctly to prevent further damage to its leaves and stems.

Change the location of your plant: If your plant is being over-exposed to harsh sunlight and temperature variations, it may develop a reddish coloration. You can change the location of your plant to prevent it from getting a reddened appearance. It is advisable to place your plant in a place that is shaded by trees or walls.

Increase the amount of fertilizer: Once you have pruned your Hydrangea, fertilizing it will help to promote new growth. Nutrient deficiency (or the incorrect pH for the plant) would be noted in a soil test report and, subsequently, application of the appropriate fertilizer to encourage new growth and recovery.

Following are general articles concerning the care of Hydrangeas:
https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/general-care-hydrangeas
https://ohioline.osu.edu/factsheet/hyg-1263
https://extension.illinois.edu/blogs/garden-scoop/2021-03-20-hydrangea-care

What to do with Hydrangea Leaves Becoming “Pale”

There is not so simple answer as to why the leaves are also “turning pale”, because I do not know if you mean “pale red” or “pale green” going almost to yellow. Your second photo clearly depicts leaves which seem to be turning yellow or even white, but the veins on the leaves remain green. This may be iron chlorosis.

The cause of this disorder is a severe lack of iron in the soil, which disrupts chlorophyll formation in the leaves, reducing photosynthesis activity. Again, such a nutrient deficiency would be noted in a soil test report. The solution to the problem is the timely application of iron-rich fertilizers. For example, iron chelates, blood meal, manure, and compost contain a good level of iron for plants.

https://www.canr.msu.edu/uploads/files/Nutrition_and_fertilization_BertCreggfinal.pdf
https://hort.extension.wisc.edu/articles/chlorosis/

I believe that both of your Hydrangeas are what is known as Big Leaf Hydrangeas; this means that your hydrangeas bloom on old wood. Most shrubs that bloom on old wood should be pruned immediately after they are done flowering. Bigleaf hydrangeas bloom on old wood, which means they set their flower buds for next summer in the leaf axils of the stems the year before they bloom.

However, hydrangeas that bloom on old wood finish often flower so late in the season that pruning them after they bloom doesn’t leave enough time for them to regrow and form flower buds for next year. If this is the case with yours, avoid pruning this type of hydrangea except to remove any completely dead branches in the spring once the plant starts to leaf out.

Some of the newer bigleaf hydrangeas are rebloomers. . . which means that in addition to blooming on old wood, they also form more flower buds on new growth and bloom a second time that season. You would know better than I what type you have.

https://extension.umd.edu/resource/pruning-hydrangeas/arec.umd.edu/resource/pruning-hydrangeas

https://extension.illinois.edu/blogs/good-growing/2022-03-10-how-properly-prune-hydrangeas

Concerning whether to add additional iron chelate:

Chelated iron is a product typically used to reduce symptoms of iron deficiency chlorosis on many kinds of plants. If we ignore for a moment the fact that iron deficiency is almost always caused by incorrect pH levels, you should know that too much iron can also be a problem. An overdose of iron may cause symptoms of iron toxicity in various trees and plants, such as stunted tops and root growth and foliage that is dark green or dark brown to purple.

Application technique nearly always involves spraying the foliage and stems of plants.

Below is a link to a Materials Safety Data Sheet that details toxicity and much more:
https://southernag.com/wp-content/uploads/2020/06/Ch-Liquid-Iron-1.pdf