Why is the moon red during eclipse?
Earth's atmosphere provides an orangish color to the moon during an eclipse. The atmosphere acts like a filtered lens. Particles in the atmosphere cause the light rays coming from the sun to bounce around. Some are refracted, or bent. They get redirected through the atmosphere and out around behind Earth and onto the moon, which is blocked only from direct sunlight. It bends red sunlight into our planet's shadow and scatters out blue light. The refracted rays of sunlight doing the illuminating turn the moon a strange reddish.
Why is it red but not other colors?
Red light travels in a straight line more easily than other colors. So when the moon is starting to enter the shadow of the earth, the light that is traveling close to the earth on its way to hitting the moon is going through our atmosphere. And while its doing this, the other colors within the light are getting spread around and less of it is hitting the moon, but the red light isn't spreading around as much and more if it is hitting the moon, so more red light is hitting the moon than any other color from the sun's light.
It's the same reason why sunrises and sunsets appear reddish. If Earth were an airless planet, its shadow would be pitch black and the eclipsed moon would be invisible. Earth would appear as a dark disk surrounded by a brilliant red ring-our atmosphere glowing with the light of all the planet's sunsets and sunrises. It's this light that we see bathing the moon during totality.