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Watering the Knock Out roses is not really any different than the other rose bushes. These articles should help: https://www.gardeningknowhow.com/ornamental/flowers/roses/watering-roses.htm https://www.gardeningknowhow.com/ornamental/flowers/roses/water-roses-droughts.htm

It very well could be the voles. Typically, you will not see anything on the surface of the ground. They burrow around underground and feed on the roots. Without their root system, the roses cannot support the top growth, thus it wilts and dies. This can be a slow death too as the voles destroy/eat more and more of the root system of each rose bush. This article should help: https://www.gardeningknowhow.com/plant-problems/pests/animals/vole-control.htm

Unfortunately this happens with most yellow roses when the weather gets hotter. In the cooler spring temps and early fall temps the yellow color is typically richer and lasts longer. You can also add some extra potassium to the soils as it will help build the root system and enhance the color in roses and other flowers. If you have never given your roses Kelp Meal, that is another thing you can give them that helps them fight stresses and disease, along with providing some great nutrients. The stronger the rosebush is, the brighter the bloom color and overal vigor of the rosebush.

Usually the blooms that are more dull in color are the older blooms that are just about done. The more vividly colored blooms are usually the newer blooms that have just opened up or have only be open a day or two. When they are just about done with their bloom life cycle the color gets more dull, then the petals fall off. More buds are usually already forming by this time to bring on new blooms. If some new blooms are bright and some not, then it could be a problem with a fungus or virus affecting the rosebush.

If you have a virus in the roses, there really is not much you can do. The viruses that I am aware of will typically stunt the growth and make markings on the foliage. Some will eventually kill the rosebush. If it is just funguses you are dealing with, then an earth friendly fungicide named Green Cure works very well to cure and keep funguses away. There are also some others on the market such as Banner Maxx or its generic form named Honor Guard. For black spot that is out of control, Mancozeb works the best.

Adding some good old sphagnum peatmoss will help get the soils more acidic. However you want to do a pH test to be sure where the soils pH level is first. Also adding some rabbit manure to the soils will bring the acid level up as well as provide some soil building nutrients to the soils. For a quick hit of acid if you are sure it is low, water the roses with some Miracid by Miracle Gro.