It is intended to provide a flower color crossing method whereby a specific flower color is passed to the next generation of delphinium and a flower color crossing method whereby a dichromatic flower color is passed to the next generation of delphinium. It is also intended to provide a method whereby perpetual delphinium can be efficiently obtained in a warm place and a method of determining a delphinium flower color based on the ratio of main inherent colorants in the sepal. It is found out that a specific flower color can be passed to the next generation by using a full-color delphinium as a pollen parent or a seed parent and carrying out allogamous crossing and, at the same time, a dichromatic flower color can be passed to the next generation thereby. By growing a delphinium seedling under such conditions as allowing its germination in a petri dish at a temperature of about 15.degree. C., a method of efficiently obtaining perpetual delphinium in a warm place is found out. By analyzing delphinium flower colors and inherent colorants, a numerical formula for determining a delphinium flower color is established. A crossing method for obtaining purple or pale purple delphinium flowers containing novel anthocyanin as the main component is found out. Also, a method of isolating and purifying the novel anthocyanin colorant is found out.