Why are several coloured filters used to search for quasars?

Astronomers use multiple coloured filters to search for quasars primarily because these objects appear unusually blue compared to normal stars. By photographing large areas of the sky through two or more different-coloured filters, researchers can compare the images to isolate these distinctively blue objects.1

This technique remains a primary method for identifying quasars, as the specific combinations of filters are effective at isolating them from other celestial bodies. Furthermore, the use of multiple filters allows astronomers to identify quasars at different redshifts, as the light from these distant objects is shifted toward longer, redder wavelengths due to the expansion of the universe. Once these candidates are identified, their nature is typically verified through subsequent spectroscopy.12