Where do all the striking colors come from we enjoy when diving in a tropical reef? It is all about light and chemistry! But things are not so easy to understand, and I thus divide my answer into several chapters. Basically, when we see a color, this is the result of light originating from a light source (the sun, a dive lamp or whatever), travelling and being changed (absorbed, reflected, diffracted, polarized to name some effects) in a medium (e. g. water and a colorful fish), and finally hitting a detector, which is a photoreceptor cell in the retina of our eye. This is how vision works and how a scientific spectrometer works: all we need is a light source, a medium that changes the light, and a detector that is sensitive to this light.