Summer is just between spring and autumn.

Spring blossoms and autumn bears fruit, so summer is the most vigorous time for growth, that is, "between spring and autumn" and "the peak of life alternation"

Moreover, the author defines spring as green and autumn as red, and the transition between them can naturally be yellow, that is, "connecting the past and the future, life alternates", the natural spectrum is "red, orange, yellow, green, blue and purple", and the transition between red and green is yellow.

This is the author's intention.