Angels will get a slight tilt to them if there is a little ammonia or nitrite present, I would do a quick water test. Often this is just their behavior, I've noticed that thet seem to think they can get a little more speed to them if they get a little tilt to them before they charge across the tank. If they are constantly tilted to one side this may indicate a swim bladder problem.
The darkening & lightning of color is normal, silvers will usually look a little pale when stressed, and color up when contented. They are generally lighter in the morning when you turn on the lights, and color up when feeding. This is normal for many fish.
Medicating well fish is how resistant strains are formed. There are very few times a well fish should be medicated, this usually involved wild caught fish, or large hatchery operations. Angels are not prone to any certain disease any more than other fish, but do need clean water. Larger and/or more frequent water changes are a good thing to get in the habit of with angels.
