The bottom of your socks can be brown because of food ulcers or diabetic foot ulcers or a sore on your foot causing blood to stain the sock which can look brownish when it dries.
Most often you can get the brown off your socks by using baking soda and a tub of warm water.
To whiten your socks with baking soda, soak your dirty socks in a tub of warm water mixed with baking soda.
Leave them for a few hours before washing them in the laundry with your regular detergent mixed with a little more baking soda.
You should wash socks twice if they are really dirty, although washing socks once is most often good enough to get them clean and get rid of bacteria and fungus.
Socks should be washed after each use.
When you wear socks you can get sweat and bacteria in the socks and wearing the same socks longer than a day or not washing the socks between each use can cause your feet to get bacteria and fungus and even rashes on your feet.
Wearing dirty socks even though they may seem clean can also make your feet smell and your feet hurt.
Even if your feet aren't sweaty, you shouldn't really wear the same pair of socks two days in a row.
The reason being, is your feet contain around 250,000 sweat glands and can excrete as much as half a pint of moisture a day.
Bacteria can thrive on dirty feet and grows rapidly in warm, damp socks.
A bacterial infection can lead to painful wounds, possibly even be life-threatening.
Typically, socks can last between six months to a year if worn once a week.
Socks should be changed regularly to avoid unpleasant smells and athlete's foot (a fungal infection).
Fungus loves warm, damp and dark environments which makes your socks the ideal breeding place.
So you should change your socks at least once per day or more often if your feet are sweating a lot or they get wet.
Wearing socks to sleep comes with very few risks, other than discomfort for those that prefer the feeling of being barefoot.
Be sure to wear socks that are loose, and do not wear compression socks to bed unless directed by a doctor or other medical professional.