Let us take a last look at the Hinomaru. The strong contrast between the white background (Yayoi) and the dazzling red, dynamic sun (Joomon) is unmistakable. Yet an astonishing chemistry unites the two elements of contrasting beauty. If we regard the white portion as virginal and the red circle as blood, the Hinomaru becomes a metaphor for marriage, the marriage of fire and water, as the spring of beauty and its infinite variety – that is the true face of Japanese tradition, as embodied by the Hinomaru.

(here end the passages of the very nice book
Kirei – Posters from Japan
Koichi Sato: Hinomaru, the Circle of the Sun)