JAMES M.GRICHNIKMD, PhD
Color variation of melanin depends on its location in the skin. In the stratum corneum and upper epidermis, melanin is jet black (appearing dermoscopically as dark spots); in the basal layer and dermoepidermal junction, it is brown (brown globules); in the papillary dermis it is blue-gray; and in the reticular dermis it is steel blue (blue nevus). Due to the Tyndall effect, in which short-wavelength visible light (blue) is dispersed and reflected more than long-wavelength light (red), the blue color of otherwise black melanin is explained by the depth of the pigment deep in the dermis. The dermoscopic image of a blue nevus in Figure 1A shows color perspective with change in color to blue and fading color intensity (pale blue) due to the Tyndall effect. The histopathologic image in Figure1B shows melanocytes deep in the dermis.
Weismann K, Lorentzen HF. Dermoscopic Color Perspective. Arch Dermatol. 2006;142(9):1250. doi:10.1001/archderm.142.9.1250