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The red leaves can mean just new foliage coming out. If it is mottled looking and the foliage looks out of character with the rest of the bush, this may be something to keep an eye on as it could be a virus called Rose Rosette and there is no cure for it. If the bushes get it, the best thing is to destroy them and buy new rosebushes. The white spots may be mealy bugs.

Here are a couple links to articles for you:
https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm

Check to see if anything has been inadvertently poured on the ground near the rosebushes that could have burned the root system. Be sure the rosebushes soils are not too wet. Soaked soils do not allow proper oxygen movement into the soils and will choke off and kill a rosebush. Look the bushes over well and see if there are any insects causing problems, such as Thrips or Spider Mites. Check the foliage over well to see if any funguses are attacking the foliage. Here is a link to a website where you can visually identify some of the problems with roses: http://www.missouribotanicalgarden.org/Portals/0/Gardening/Gardening%20Help/Visual%20Guides/Rose%20Problems.pdf . It is best to positively ID what a problem may be so that the right course of action is taken.

Here is another link for you as well: https://www.gardeningknowhow.com/ornamental/flowers/roses/no-blooms-on-roses.htm

Here is an article link on Knockout roses too: https://www.gardeningknowhow.com/ornamental/flowers/knockout-roses/care-knockout-roses.htm

It could be that the fertilizer being used is too high in nitrogen. Repeated applications of the Miracle Gro Liqua Feed and others like that can lead to lots of great foliage but either no blooms or next to no blooms. Check out this article link too: https://www.gardeningknowhow.com/ornamental/flowers/roses/no-blooms-on-roses.htm

I would not prune the rosebushes back more than 4 to 5 inches at this point. They are probably very stressed out by the Japanese Beetles attacking them, they need to be kept watered well. I recommend watering them with some water that has a product called Super Thrive mixed into the water along with a little water soluble fertilizer, not as much as you might typically use but about half that amount on the fertilizer. This will help them deal with the shock and stress they are currently under. I would also recommend spraying the rosebushes with either Bayer Complete insecticide or Sevin to help lessen the Japanese Beetle problems.

The white spots could be Mealy Bugs or perhaps Spider Mites. They could also be the beginnings of Powdering Mildew or could be dust particles from something being applied in a neighboring property. Could also be ash from a fire if there have been any in the area. The action to take is to see if the spots can be dusted off with just a flick of the foliage, or if the spots move when you touch the foliage. If they move then then are likely mealy bugs or spider mites, an application of Insecticidal Soap mix should gain control. If the spots do not dust off and do not move, it might be a good idea to spray the rosebushes down with a good fungicide such as earth friendly Green Cure. You can purchase Green Cure on-line.

Here is a link to an article that you should find very helpful for all of your questions. If you have more questions after reading the article, please feel free to ask them here as well. The Knockout roses winter care is really no different than other rosebushes. Here is the link: https://www.gardeningknowhow.com/ornamental/flowers/roses/roses-winter-care.htm