All the 7 Rainbow colors representing each day of creation and when combined produce white color. It is a known fact that shining white light through a prism produces the rainbow colors. White signifies purity and goodness. No wonder on the 6th day, God declared that everything he has created during the 6 days of creation as very good and blessed it on the 7th day.
Genesia1:31 God saw all that he had made, and it was very good. And there was evening, and there was morning—the sixth day.
Genesis2:1 Thus the heavens and the earth were completed in all their vast array.
2 By the seventh day God had finished the work he had been doing; so on the seventh day he rested [a] from all his work. 3 And God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done.