Bismarck brown Y is a small molecule chemical dye.  It is composed of a central benzene ring attached to two other benzene rings via azo groups, or nitrogen nitrogen double bonds.  Because of all of these aromatic pi cloud electrons, bismarck brown Y has interesting light absorbance properties.  As a result, the chemical can absorb all wavelengths of visible light except a combination that produces the color brown.  Brown is a tertiary color resulting from a mix of three colors cyan, magenta, and yellow.  So the other colors are absorbed and the mix of brown is reflected from the chemical, thus giving the brown appearance.