Using a black permanent marker, draw a rainbow outline on a clear acetate sheet or transparency film. A rainbow has 7 sections, but I only made 6 on mine.
You can also print out this rainbow template on A4 or Letter size paper. Place the paper under the clear acetate sheet and use it as a guide to draw a rainbow outline on the plastic sheet.
For a transparent colored rainbow, use glass paint. You can buy it from craft stores or make your own glass paint. For this tutorial, I actually used acrylic paint, which is somewhat transparent when applied thinly.
Prepare the 3 primary colors—red, yellow and blue. Mix the primary colors by pairs to come up with the 3 secondary colors—orange (red + yellow), green (yellow + blue), and violet (blue + red). The rainbows's 7th color is indigo, which is a mixture of blue and violet, with blue being more dominant.