I dislike with passion the answer "because Rayleigh scattering". When someone asks why, especially if a child asks, the default answer should be the simplest correct answer:
Because it's the color of the atmosphere, specifically nitrogen and oxygen! It's technically correct to state this.
Gasp! But aren't nitrogen and oxygen usually described as "colorless"? Well, yes but... If they were perfectly colorless, the sky would be black. It's technically more correct to describe them as nearly colorless and very slightly blue. Very slightly because you need to see through kilometers of atmosphere to perceive the blue. It doesn't matter if the color is caused by absorption, or reflection, or (Rayleigh) scattering of certain wavelengths. The "color" of an object is simply the color you perceive with your eyes. If you perceive blue, it's technically correct to say its color is blue.
It's like saying plants are green because green is the color of chlorophyll. And in the case of chlorophyll, the color is caused by absorption not by scattering. But the physics is irrelevant. Green is its color.
Q: But sunsets/sunrises are red & orange not blue! A: the simplest answer is: color of an object can change under different light conditions. Specifically in this example, when seeing the sun through not kilometers but hundred of kilometers of atmosphere, all the blue-ish wavelengths have been scattered in random directions so only the red-ish wavelengths remain, thus the atmosphere is illuminated by progressively redder and redder light as the photons travel longer and longer distances through the atmosphere.
At least my mental model of color before I learned the details in physics and chemistry was that of a property of surfaces, so your explanation wouldn't have served me. I would have expected, that the light becomes blue as part of the transition from "empty space" into "air", not as part of the "flight" through the air.
You can conceptualize Rayleigh scattering exactly like you did so it doesn't conflict with my explanation: as light hits the "surface" of nitrogen and oxygen molecules, it "reflects" (scatters) blue wavelengths.
I was talking about the understanding of a child, which didn't learned physics and chemistry yet, so there are also no atoms. Of course now I know about them, but at the same time, what I used to call a "surface", simple doesn't exist anymore.
I'm surprised that there were downvotes. This is an excellent answer, and better interfaces between linguistic definitions of color and physicists' than saying "Rayleigh scattering impacts blue more than red"!
Because it's the color of the atmosphere, specifically nitrogen and oxygen! It's technically correct to state this.
Gasp! But aren't nitrogen and oxygen usually described as "colorless"? Well, yes but... If they were perfectly colorless, the sky would be black. It's technically more correct to describe them as nearly colorless and very slightly blue. Very slightly because you need to see through kilometers of atmosphere to perceive the blue. It doesn't matter if the color is caused by absorption, or reflection, or (Rayleigh) scattering of certain wavelengths. The "color" of an object is simply the color you perceive with your eyes. If you perceive blue, it's technically correct to say its color is blue.
It's like saying plants are green because green is the color of chlorophyll. And in the case of chlorophyll, the color is caused by absorption not by scattering. But the physics is irrelevant. Green is its color.
Q: But sunsets/sunrises are red & orange not blue! A: the simplest answer is: color of an object can change under different light conditions. Specifically in this example, when seeing the sun through not kilometers but hundred of kilometers of atmosphere, all the blue-ish wavelengths have been scattered in random directions so only the red-ish wavelengths remain, thus the atmosphere is illuminated by progressively redder and redder light as the photons travel longer and longer distances through the atmosphere.