Why does my inner tube have brown liquid inside? Now with internal photo
I've seen that color before. If you are filling from a compressed air tank, make sure the tank has been bled recently. More frequently in humid areas air tanks will get condensation inside. Normal maintenance is to bled the water out of the tank. The condensation rusts the inside of the tank and can eventually cause tank failure. When you put an air line on them the air comes out but a little vaporized water goes too. That gets in your tubes and builds up over time if you use the same filling source. Avoid by placing an in-line filter on your air hoses as well. Test by blowing air over your hand from a safe distance. If it's wet or damp you have found the culprit.

Experience case: They didn't bleed the compressor at the beach shop I worked at. I bled it when we upgraded to prep it for storage. One gallon of water came out of that little six gallon tank. One-sixth of the volume for air was taken up by the water and it was the cloudy brown milky color too.

Also, respectfully agreed. White liquid is often stop flat liquid filled into a tube as a flat preventative. Stop flat can be neon green in color too.