Final answer:
The colour of light from an accretion disc varies due to temperature differences within the disc, leading to emissions ranging from X-rays to visible light. Timescales of light variation in quasars suggest the size of the accretion disks. Scattering by dust and gas also causes changes in the observed colours of light.
Step-by-step explanation:
The colour of light from an accretion disc varies due to the different temperatures and velocities within the disc, which produce a range of electromagnetic radiation. Accretion discs around large astrophysical objects, such as black holes, contain gas and dust that heat up as they spiral inward. The inner parts of the disc are hotter and can emit X-rays, while the outer, cooler parts emit light in the visible spectrum. This variation in temperature leads to the emission of various colours of light. Moreover, quasars, which are believed to be powered by accretion discs around supermassive black holes, exhibit changes in the visible light on timescales of 5 to 2000 days. These variations limit the size of the disc to light-days across, corresponding to the time taken for the light to travel across the disc.
Additionally, the phenomenon known as scattering can cause the colour of light to change. For instance, interstellar dust scatters blue light more efficiently than red light, which makes distant stars appear redder than they are, a process that parallels why Earth's sky looks blue. This principle also applies to light from an accretion disc passing through surrounding dust and gas, resulting in a variation of observed colours depending on the angle and the presence of nearby matter.