The information coded in the sample viewer is on a per channel basis. This means, you'll have to split in your mind the three RGB channels
Basically, for a grayscale image, 0 (black) means that this particular pixel will not be included as a valid source for background estimation. 1 (white), by the other hand, means that is a perfectly good sample. Intermediate values means partial inclusion on the calculation (the factor may be considered as a weight).
So, the same happens on RGB channels, but there is no correlation between different channels. A red pixel means that only the information on the red channel has been included. A yellow one, red and green pixel values.
