This article represents an analytical formulation for optical properties of absorbing metallic nanoparticles in visible region of electromagnetic spectrum based on Mir theory, in order to find a clear relationship between fundamental characteristics of nanoparticles with their spectral behavior and color coordinate in CIE LAB color space. Calculations were performed on nano particles with various diameters (d = 50, 100, and 200 nm), as well as complex refractive index with different real (11= 1.5, 2.0, 2.5, and 3.0) and imaginary (k = 0.001 and 0.1) parts. Obtained results reveal that scattering phenomena in nanoparticles are strongly linked to the particle size parameter and complex index of refraction. The results indicate that the reflectance and lightness (L*) of nanoparticles increases as a result of increase in their size and real part of complex refractive index and decrease with increasing the imaginary part of complex refractive index. The CIELAB colorimetric system was used for analysis the color of nanoparticles. According to obtained results, all nanoparticles have greenish-blue color, and undergo color change as a result of varying in their size and the complex refractive index. (C) 2015 Wiley Periodicals, Lac. Col Res Appl, 41, 477--483, 2016; Published Online 26 August 2015 in Wiley Online Library (v,ileyonlinelibrary.com).