A cloud that might appear white when it's in front of a black cloud will look gray if clear sky is behind it. That is, they send the blue light in all directions while more of the other colors continue on to the ground. The scattered blue light is scattered in all directions in the sky. Blue light that comes in our direction makes the sky look blue. When the sun is low in the sky at sunrise and sunset, the light has to go through more of the air.
More blue light is scattered out but almost all of the red and yellow light reaches our eyes.
When a lot of small particles, such as dust or air pollution, are in the air, the sky is even more red because these particles scatter even more of the blue light. A University of Illinois World Wide Web site has photos and more information on sunset color. Click here to go to this site. They are seen at a 22-degree angle to the sun. Cirrus clouds, which are made of ice crystals, are generally in the sky to create sun dogs.