The seven colors of a rainbow are: Red, Orange, Yellow, Green, Blue, Indigo and Violet.
A Rainbow template (for PowerPoint) is a template containing a representation of a rainbow. It can be a colorful template containing the seven colors of a rainbow or a rainbow diagram with a visual representation that you can use in a PowerPoint presentation.
Free rainbow PowerPoint templates are nice backgrounds with representations of a rainbow in the sky and useful for presentations. Alternatively you can download other free colorful PowerPoint templates.