To make one, or many, start by cutting strips of tissue paper in your chosen colors. I went for the classic ROY G. BIV, without the indigo, because I think blue and purple pretty well cover that end of the spectrum; no need to get all medieval with the coloring.
Cut the tissue paper into skinny strips; mine were about a third of an inch wide, and two to three inches long. Use a brush to apply matte Mod Podge to one end of the cardboard, and lay the first tissue paper strip on top, allowing the paper to hang off three sides of the cardboard. Brush on another stripe of Mod Podge, and overlap the second strip on the first.