Colored grains including red, purple, and black rice, purple corn, black barley, and black soybean contain anthocyanins. The present study was designed to (i) identify and quantify the individual anthocyanins and measure the total phenolic content (TPC), (ii) evaluate the antioxidant and alpha-glucosidase inhibitory activity, and (iii) correlate the TPC with total antioxidant activity and alpha-glucosidase inhibitory potency in these colored grains. The TPC was measured using a Folin-Ciocalteu assay, while the total antioxidant activity was determined by a method based on the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity. Among all of the studied colored grains, black rice possessed the highest TPC, which was 86 times greater than that of red rice. In addition, black rice had the highest total anthocyanin contents and alpha-glucosidase inhibitory activity. A significant positive correlation of the antioxidant activity and alpha-glucosidase inhibitory activity with total anthocyanin content and TPC was observed in this study. It is concluded that black rice possesses the highest antioxidant activity and alpha-glucosidase inhibitory among all of the colored grains tested and can be further explored as a functional food.