The term 'Part of a Rainbow' can refer to two different crossword solutions: INDIGO (6 letters) and HUE (3 letters). INDIGO is a deep blue-violet color that is one of the seven colors of the rainbow. It is located between blue and violet in the visible spectrum. HUE is a term used to describe the gradations of color, such as red, orange, yellow, green, blue, INDIGO, and violet. It is also used to describe the intensity of a color, such as light or dark. Both INDIGO and HUE are part of the visible spectrum of light and are therefore part of a rainbow.