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Anthocyanins, soluble sugars, and organic acids play a vital role in the color and flavor of grape berries. N and KH₂PO₄ are essential nutrients for grape growth and development. However, the research on the effects of foliar spraying of KH₂PO₄ on the skin color and flavor of grapes under different N levels were not systematic. In this study, “Flame seedless” grapes were used as the test material. There were six treatments in this experiment, including low nitrogen (LN), low nitrogen + KH₂PO₄ (LNK), moderate nitrogen (MN), moderate nitrogen + KH₂PO₄ (MNK), high nitrogen (HN), and high nitrogen + KH₂PO₄ (HNK). Foliar spraying of KH₂PO₄ on grapes significantly increased total K, anthocyanin contents, and the color index of red grapes (CIRG) in LN, MN, and HN. In the N and KH₂PO₄ treatments, foliar spraying of KH₂PO₄ significantly increased the content of methylated, acetylated, and coumarylated anthocyanins under MN treatment. The glucose and fructose contents of MNK were the highest compared to other treatments. The sole use of N showed the highest glucose and fructose contents with MN application. Anthocyanin had a significant positive correlation with soluble sugars; and showed a significant negative correlation with organic acids. Overall, foliar spraying of 0.5% KH₂PO₄ improved the color and flavor of “Flame seedless” grapes under all N levels, with the most significant effect at MN.