How the primary colours of light combine.
Credit: SharkD

In physics colour has a specific meaning. We usually associate colour with the way our eyes see objects, where the colour is our way of understanding the particular wavelengths of light which are reflected from an object. We see white where all of the visible light is reflected, and black were all of the visible light is absorbed. The colours we see in between are all different combinations of the visible light which is reflected. Visible light is a small portion of the whole electromagnetic spectrum which our eyes are designed to see. It covers a wavelength range from 400-700 nanometres, where a nanometre is a billionth of a metre.

In astronomy we think of colour as the difference between the brightness of an object when we look at it in two different bands where a band represents a colour, for instance the b-band would be a blue colour (or wavelength range), and r-band would be a red colour (or wavelength range). When we take observations of objects we place a filter in front of the camera on the telescope to get the light in that colour or band. When we see colour images in astronomy they are made by combining the observations in these different bands together, and we call them either false colour or representative colour images, which we can create by combining 3 different bands of light.