If you need some kind of rainbow effect, the wavelenth node is great. Here’s an example where I created a very simple confetti like material. This uses Voronoi Cells.
The main thing to notice is that wavelengths are large numbers. Textures produce numbers between 0.0 and 1.0. You’ll need to use a Math node set to multiply, or else you’ll just get black.
Also, if you don’t want black in your result, you should either Add to the textures output, or use Less Than to create transparency for low numbers.
Here’s a reference for wavelengths:
Play around with different textures, or even use images.