I have an '07 Focus shop manual, the procedure to fully change fluid is disconnect the cooler return line, it has a disconnect coupler, and add a bit of extra hose to make life easy. Then start the engine let it idle for around a minute to pump out a measured 3 quarts/liters(into a marked container) add 3 qt, repeat thrice to a total of 9-10 qt. Do not operate above idle rpm, probably best to work with a warmed up engine. The recommended fluid in "Mercon V".
The return line method has always been the most effective as it results in the least dilution of new fluid with old. I used this method with my old '79 Malibu 350/350 (highschool/first car), the change from old slightly dark fluid to new color comming from the return line was very sudden. It went from barely shifting when cold to like new shifting. (it had a lot of hard miles on it when i got it).
The Ford manual recommendation is 30k miles for all Torqshift transmissions, 60k for CVT trans, and 150K for all others under "normal" driving --otherwise they recommend 30k if you do anything other than nice 50mph open flatland traveling with no cargo or roof rack, in mild spring weather, with a tailwind, while on a diet...
Darkness is caused by scorching or friction dust, however the friction additives breakdown even without significant darkening.
I have noticed Ford has had a tendency over the last few decades to make maintenance recommendations that seem designed to limit the life of the vehicle. Such that everything is worn out just after warranty. eg don't change the brake fluid, don't change the transmission fluid, deemphasize coolant changes, etc. Corrosion kicks in and the next owner needs to replace major parts or send it to the wrecker as the block rusts through, and the wrecker can't even sell remaining parts because of the same issues.
I bought my [used] '07 Focus SE(the mid model) because it was dirt cheap and I had to commute to college during winter. (350cc motorcycle for summer) But it is a rattle trap with poor power, poor fuel economy(for its size), poor handling, flimsy feeling controls, vibration, and a weakas horn sounds like a sick donkey. My girl's '02 Camry SE is noticably superior in every aspect, even maint., with one exception that I can put a bicycle in my hatchback.