To investigate the genetic diversity of key grain traits in colored hulless barley germplasm resources from the Qinghai–Tibet Plateau, this study analyzed 122 colored highland hulless barley accessions collected from Qinghai, Tibet, Sichuan, Gansu, and Yunnan. Six grain traits (thousand-grain weight, grain length, grain width, grain thickness, grain area, and roundness) were evaluated at four locations within a single year. A comprehensive analytical approach was employed, including genetic diversity analysis, correlation analysis, principal component analysis, cluster analysis, and comprehensive evaluation of grain traits, to identify and screen elite germplasm resources. The results showed extensive phenotypic variation across all six grain traits. Thousand-grain weight exhibited the highest coefficient of genetic variation, while grain width showed the lowest. The Shannon diversity index for each grain trait ranged from 1.95 to 2.09. All grain traits were significantly influenced by environment, genotype, and their interaction. Correlations were commonly observed among grain traits, with a correlation coefficient of 0.891 between thousand-grain weight and grain area, and a negative correlation coefficient of -0.693 between grain length and roundness. The first two principal components (PC1 and PC2) accounted for a cumulative contribution rate of 94.073%. Thousand-grain weight, grain width, and grain thickness were the three most influential traits in PC1, while grain length and roundness were the main traits in PC2. Cluster analysis classified the 122 accessions into four groups. Group I, comprising 32 accessions, exhibited the best overall trait combination, characterized by maximum values of thousand-grain weight, grain length, grain width, grain thickness, and grain area, representing a typical large-grain type. These accessions can serve as parents for breeding new high-yielding hulless barley varieties. Based on the comprehensive score (F value), the top ten accessions—Ziqingke (Sample designation 2), Chaqingmeicha, Xiajiaozhi, Chajiu, Hunzai Naiqingzhong, Haiqinghei No. 2, Wumu, Naichai, Naina, and Chedouzi—were selected as elite parents for improving grain traits and screening new varieties of hulless barley germplasm resources.