I'm glad your turtles don't look like turtle #1, who has a severe bacterial shell infection.
Severe shell infections will soften the shell and are often (but not always) smelly and/or wet. There may be some bleeding. They tend to get their foothold along the seams of the shell, or at the site of an injury, or underneath seriously retained scutes.
Bruises on a shell can look dark. Fungus usually looks bright white. Things like iron in the water or color on a basking platform can stain a shell. Minerals in hard water can coat a shell with a white glaze. When a scute is getting ready to shed its top thin layer, it gets a dull, whitish look.
It's also possible that your turtles' shells are perfectly healthy, and the brown you are seeing is just part of the normal pattern or some sort of minor stain.
If you are concerned about your turtles' health, and I suppose you are since you posted here in the Urgent Care section, you should make an appointment with a good herp vet and take them in for a check-up. Since we can't see the problem, there's not a whole lot we can do to help.
Here's a caution if you see a vet. If the vet offers to give a vitamin A shot, first make sure the vet is positive your turtle really needs vitamin A and then discuss other alternatives to shots with the vet like eye drops with vitamin A, cod liver oil or food supplements. Several turtles have recently had very bad reactions to vitamin A shots and I think it's best that if vitamin A is needed (and often it's not) the safer methods be used instead.
Also, when you clean your turtles' shells, be very gentle. There are nerves in the shells and especially along the seams they are sensitive. If you use a brush, make sure it's soft and brush lightly. You can also clean the shell with a wet paper towel.
