Most astronomical instruments available today only measure the brightness or intensity of things - they are not able to measure colour at the same time.
However, colour can tell astronomers a lot about the universe - for example hot, blue stars are usually bigger and brighter than small red ones.
To find out about these colours, astronomers use special coloured glass filters. These only let through light of a particular range of colours, for example red.
By looking at the differences between images taking using filters, you can find out a lot about the colours of the objects - for example, a blue star will be brighter in an image taken through a blue filter than in one taken through a red filter.
Filters on professional telescopes are very carefully made to let through an exact range of colours. This means that filters on different telescopes are the same and so astronomers can easily compare images from different telescopes.
Examples of some filters: