Light passing through a prism is refracted or divided into the colors of the spectrum.
So we have not only the colors of the major four archangels but of the others also. Black is the absence of color, meaning the powers of darkness are not covered by the protective shield of God. We might also add the analogy of Noah's ark and learn its spiritual lesson. Noah and his sons represented the four archangels, and the ark itself signified their saving power. The rainbow rose is a shield over the heart.