Spectrophotometry approximates eumelanin and pheomelanin better than chemical methods for large studies. Green-to-red reflectance ratio best distinguishes red from nonred hair. MC1R genotype explains some but not all red hair variation. Red-spectrum light absorption best captures light-dark hair continuum. Genetic influences on light-dark continuum differ qualitatively between males and females. Data from 1730 adolescent twins. Reflectance curves and subjective ratings measured hair color. Discriminant analysis validated spectrophotometric proxies. Eumelanin drives dark hair, pheomelanin drives red hair. Variance components show heritable basis for both traits.