Canon EOS 6D Instruction Manual page 241

B: Shooting Information Display
[RGB] Display
This histogram is a graph showing the distribution of each primary
color's brightness level in the image (RGB or red, green, and blue).
The horizontal axis indicates the color's brightness level (darker on
the left and brighter on the right), while the vertical axis indicates
how many pixels exist for each color brightness level. The more
pixels there are toward the left, the darker and less prominent the
color. The more pixels there are toward the right, the brighter and
denser the color. If there are too many pixels on the left, the
respective color information will be lacking. If there are too many
pixels on the right, the color will be too saturated with no gradation.
By checking the image's RGB histogram, you can see the color's
saturation and gradation condition, as well as white balance
inclination.
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