Because each pixel has a red, a green, a blue and an alpha component you can separate them. You can split an image into four, one containing only the red intensity, one containing the Green, one the Blue and one the Alpha. You can only do this for raster images with a depth of 32 bits.
In the image below we show an original image and three additional images, one for each color channel. Note how for the red channel for example, the red parts are almost white because have a higher intensity. Remember the higher the intensity the closer to white.