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We present a systematic algorithm capable of searching for optimal colors for any lightness L * (between 0 and 100), any illuminant (D65, F2, F7, F11, etc.), and any light source reported by CIE. Color solids are graphed in some color spaces (CIELAB, SVF, DIN99d, and CIECAM02) by horizontal (constant lightness) and transversal (constant hue angle) sections. Color solids plotted in DIN99d and CIECAM02 color spaces look more spherical or homogeneous than the ones plotted in CIELAB and SVF color spaces. Depending on the spectrum of the light source or illuminant, the shape of its color solid and its content (variety of distinguishable colors, with or without color correspondence) change drastically, particularly with sources whose spectrum is discontinuous and/or very peaked, with correlated color temperature lower than 5500 K. This could be used to propose an absolute colorimetric quality index for light sources comparing the volumes of their gamuts, in a uniform color space.

In spite of color being one of the physicochemical parameters most commonly used to characterize ornamental stone, there is yet no standardized protocol for measuring this parameter. Such a protocol is of particular importance for characterizing the color of heterogeneous surfaces, as in the case of granite. The aim of the present study was to determine the minimum area and the number of measurements required to characterize the color of granite rocks. A spectrophotometer and a tristimulus colorimeter, were used to measure the color of granite samples, and the measurements were expressed in CIE L*a*b* color system units. Three parameters were considered as variable factors: the type of rock (Labrador Claro, Grissal, Rosa Porrin˜o, and Blanco Cristal), surface finish (polished, honed, sawn, and flamed), and target area (circular apertures of diameter 5, 8, 10, and 50 mm). The results of the application of multivariate analysis of variance and of the classical CIELAB formula and CIE L*a*b*-based color-difference formulae (i.e., CIE94 and CIEDE2000) to the data revealed that, although all considered factors affected the minimal area and the number of measurements required, the different circular apertures of both the instruments can be disregarded if the number of measurements and area recommended in this study are used.

A reduced set of measurement geometries allows the spectral reflectance of special effect coatings to be predicted for any other geometry. A physical model based on flake-related parameters has been used to determine nonredundant measurement geometries for the complete description of the spectral bidirectional reflectance distribution function (BRDF). The analysis of experimental spectral BRDF was carried out by means of principal component analysis. From this analysis, a set of nine measurement geometries was proposed to characterize special effect coatings. It was shown that, for two different special effect coatings, these geometries provide a good prediction of their complete color shift.

Alumosilicate materials of different morphologies, such as platy and tubule nanoclays, may serve as an efficient, protective encasing for colored organic substances and nanoparticles. The adsorption of dyes onto the nanoclays increases their stability against thermal, oxidative, and acid-base-induced decomposition. Natural organic dyes form stable composites with clays, thus allowing for "green" technology in production of industrial nanopigments. In the presence of high-surface-area alumosilicate materials, semiconductor nanoparticles known as quantum dots are stabilized against agglomeration during their colloid synthesis, resulting in safe colors. The highly dispersed nanoclays such as tubule halloysite can be employed as biocompatible carriers of quantum dots for the dual labeling of living human cells-both for dark-field and fluorescence imaging. Therefore, complexation of dyes with nanoclays allows for new, stable, and inexpensive color formulations.