Hothesis: Because of their flexible structure and adjustable color, structural colors with non-close-packed colloidal crystal arrays (NCCAs) have broad applications. However, most of these structural colors are limited by an approximate refractive index or high background scattering, and they present an unsatisfactory color that seriously hinders their practical application. Preparation of particles with a high refractive index or adsorption coefficient may be an effective approach to construct highly colorimetric NCCA structural colors in a nonaqueous solvent. The aim of this study was to explore the formation process of NCCA by the assembly of colloidal particles in a nonaqueous solvent, so as to fabricate NCCAs with a high refractive index and high adsorption. Experiments: An attempt was made to fabricate the NCCA structural color by dispersing the desired particles in a 2-hydroxyethyl methacrylate (HEMA) solvent. Surface-functionalized poly(methyl methacrylate) (PMMA) particles were developed by emulsifier-free emulsion polymerization. The formation of NCCA was studied by comparing the dispersion of three classes of surface-functionalization of colloidal particles in the HEMA solvent. The melanin-like particles with a high refractive index and a high adsorption coefficient were synthesized by doping polydopamine (PDA) into good surface-functionalization particles by emulsifier-free emulsion polymerization. A highly colorimetric poly(2-hydroxyethyl methacrylate) (pHEMA) optical film with NCCA was fabricated by dispersing the melanin-like particles in a pHEMA hydrogel. Findings: PMMA-HEMA colloidal particles could successfully construct a NCCA structure in a nonaqueous HEMA solvent because of the solvation. Based on the "good particles," PMMA-HEMA-PDA colloidal particles, an initial 3-hydroxytyramine hydrochloride (DA.HCI) concentration of 1.926 mM was shown to significantly improve the colorimetric value of the final solidified NCCA hydrogel. These results provided an important reference for the construction of surfactant-free HEMA non-close-packed colloidal crystals. This highly colorimetric structurally colored hydrogel may be widely used in the design of a variety of colored intelligent sensors and soft devices. (C) 2020 Elsevier Inc. All rights reserved.