Although light can appear white, it is actually made up of lots of different colours all added together. The range of colours that make up the white light is called the spectrum of the light. We can talk about different parts of the spectrum, for example, the blue part or the red part, when we want to talk about particular parts of the spectrum.
You can see all the different colours by using a prism like the one in the picture, which uses refraction (the bending of light) to split the light up. This works because different colours are bent by different amounts and so they come out of the prism in slightly different directions.
Astronomers often want to know exactly what colours there are coming from a particular star or galaxy. They use special instruments called spectrographs, which separates the light and records the data.
Some of the light from stars can be absorbed (taken away) by atoms and molecules that get in the light's way. When this happens, narrow lines can appear in the star's spectrum. The spectrum of our own Sun has hundreds of absorption lines at different colours, all due to atoms in the Sun's atmosphere.