Billions of dust and water particles are constantly floating in the air. The sky gets its color from the sun, whose sunlight is a mixture of violet, blue, red, green, yellow, and orange rays, all the colors of the rainbow.
When the sun is high in the sky, these red-orange-yellow light rays stream down to earth from the sun, and we see the sun as yellow. But the violet-blue-green rays behave differently, they do not stream down directly to earth. Instead, they are mattered by the dust and water particles in the air. So when we look up, we see the blue light rays reflected by the particles, or a blue sky.
When the sun is sinking low near the horizon at sunset, the violet-bluegreen light rays follow an even longer, scattered path in the atmosphere than during the day. So we see more reds and yellows, which make the sunset look red.
Volcanic eruptions spread unusually large amounts of dust particles into the atmosphere and create even brighter red sunsets!