Study on metabolic variation in whole grains of four proso millet varieties reveals metabolites important for antioxidant properties and quality traits

•Four proso millet grains with different color were subject to metabolomic analysis.•There are more than 100 differential metabolites between white and colored grains.•Colored grains have higher phenolic acid and flavonoid contents than white grains.•Levels of amino acids are significantly different among the four varieties.