A: The "dark brown particles" you see in the fabric of your nut milk bag are the natural particles from cotton plant. When the cotton is harvested mechanically or picked by hand some of the cotton flower pedals are gathered as well -
image of cotton flower
During the manufacturing the dark flower plant particles are crushed into very small particles making it harder to remove.
Almost all pure white conventional (not organic) cotton is bleached and also dyed with toxic white dyes to give it a pure white appearance.
Since I use unbleached organic fabric there are very small cotton flower particles that did not get removed in the manufacturing of the fabric.