Why are the Sun and the Moon not the same colour?
“Why does the Sun often seem yellow, whereas the Moon looks more white? If it is just reflecting the Sun’s light, shouldn’t it appear the same colour?”
Several different optical illusions provide the Sun and the Moon with apparently varying colours over time. But the most crucial point in answering your question is that the Sun generates light, whereas the Moon only reflects it.
Sunlight is made up of all colours, and would look white if we perceived all of it. But the atmosphere scatters shorter frequencies of light (purple, blue and green) so that red, orange and particularly yellow colours reach us during daylight. If the Sun is low on the horizon, the light passes through more atmosphere on its way to us, so the longest wavelengths of light – the reddish ones – dominate over the distance, and the Sun seems redder.
The Moon’s own colour is primarily grey, and that influences the light it reflects on. Again, as for sunlight, the Moon’s light is scattered by the molecules of the atmosphere before it reaches us. So the Moon’s colour seems to change depending on its position in the sky.
But its relative light intensity also plays a role. Our eyes see the world by means of cones and rods. The rods are very light-sensitive, but not as detail-orientated. The cones are less light-sensitive, but if the light is sufficiently intense, they pick up more colours and details than the rods. So under bright sunlight things will seem more colourful than under the Moon’s fainter light, which will typically tend towards a more black and white palette.
Heavenly bodies can also display other colours such as purple and pink if particles from volcanic eruptions or wildfires are also scattering different frequencies.