There are various types of histogram in photography, including RGB, luminosity and “colors”. Lightroom uses the latter, since it shows which RGB colours are being clipped and to what extent. “Clipping” is underexposure or overexposure. It shows as data banking to the left or right of the histogram. If all three red, green and blue channels clip in unison, the tone is either pure black or white and no detail is retrievable.
In Photoshop CC and on many camera LCDs, you have the choice of viewing discrete RGB histograms. This makes it easy to see colours being clipped*, since the data is not packed into one small graph. It’s also useful for assessing colour balance, because peaks in the three histograms align when the colour is neutral. Photos with a strong colour cast or bias yield uneven histograms.
Primary and Secondary Colours
To help you understand separate RGB histograms, it’s handy to know that they each represent a range of colours. For instance, a fully saturated red will cause data to bank over to the right of the red histogram. If the data was to bank to the left it’d indicate the opposite: pure cyan. Cyan is the secondary colour that opposes red on an RGB colour wheel. Thus, you can think of these histograms as cyan to red, magenta to green and yellow to blue.
When making levels adjustments in Photoshop using the separate RGB channels, moving the left or middle slider to the right increases the secondary colour. Equally, moving the right or middle slider left strengthens the primary colour. You wouldn’t usually make these edits unless getting rid of a colour cast. Similarly, in a curves adjustment, pulling the red, green, or blue curve down boosts the secondary colour.
*If shooting raw files, the camera’s histogram does not depict exposure latitude as precisely, since it’s derived from a JPEG.