Why brows turn blue?

The cause In summary, the appearance of "blue" pigment is really an optical phenomenon, not an intrinsic property of the pigment itself. In order to understand this, one must realize that objects in the world (i.e., pigmentation of the brows) have no inherent color, and what we see is dependent on the reflection, retro-reflection, and light scattering (Mie scattering in the case of pigmentation).  From the sheer physical aspect, Channel Carbon Black's small particle size (90-100nm) reflects bluish or violet wavelengths due to its optical characteristics. However, when we add the Tyndall effect, where the unabsorbed blue wavelengths stand out, the...