At https://spaceweather.com .. on the 2nd of Novermber, we have another comet – C/2025K1, which has just survived perihelion. It was thought it might disintegrate when it rounded the Sun, four times more closely than 31/ATLAS did so a few days ago. It is now displaying a red/brown/golden hue – rarely seen in cometary comas.
At https://www.livescience.com/space/comets/theres-a-second-comet-atlas-in-our-solar-system-and-it-just-turned-gold-after-a-perilous-dance-with-the-sun … Comet C/2025K1 [ATLAS] developed a golden glow after reaching its closest point to the Sun. Until now the comet has been largely ignored – as it has been overwhelmed by 31/ATLAS, hogging all the attenntion. We also have Comet Lemmon which is clearly visible in the night sky. So, three comets in the inner solar system at the moment. Comets in the night sky are typically white. This is the colour of light and comets reflect sunlight at wavelengths of visible light. However, specific chemicals in comets can affect the colouraion of their comas [the gases being released by the comets as they interact with the solar wind]. Shedding carbon monoxide, for example, or ammonia, as in 31/ATLAS. They can either be blue or greenish in hue in those instances. C/2025K1 has virtually no carbon it might be surmised, or cyanide, suggesting the molecules necessary for those colours have already been depleted – at some stage in its life.
At https://www.livescience.com/space/comets/interstellar-visitor-3i-atlas-may-have-just-changed-color-for-the-third-time … Comet 31/ATLAS has developed a blueish hue after undergoing a rapid and unexpected brightening event whilst in the glare of the Sun. This is perhaps due to a specific gas being shed – such as carbon monoxide or ammonia. It had a reddish hue to begin with, then switched to green – and now it is blue.