May 3, 2013
To view the LAB color values of an image (instead of the RGB values), in Develop Module, Control -click (Mac) | Right Mouse -click (Win) on the Histogram and choose Show Lab Color Values. Then, position your cursor over the image preview and the values will be displayed as a LAB color readout.
June 14, 2011
Hovering your curser over the triangles in the upper left and right of the Histogram panel in Lightroom’s Develop module displays a red or blue preview of the clipped values in the image preview. To toggle on this clipping preview (so that it remains displayed as you move your cursor to other areas), tap the “J” key. The red overlay shows where information in one or more channels (R, G or B) of the image is pure white. The blue overlay shows where information in one or more of the channels (R, G or B) of the image is pure black.
To preview (more specifically) which channel is being pushed to pure black and white, Option (Mac) / Alt (Win) + drag the Blacks/Exposure sliders. Where the image preview is pure black or white, all of the channels are clipped, the various colors show which individual channels (or combination of channels) are clipped.
June 13, 2011
The Histogram displays a visual representation of the pixels in your image. You can use the Tone sliders in the Basic panel to move the values in the image or drag directly in the histogram to change these Tone values (depending on the value you click and drag on in the Histogram, Lightroom moves the appropriate Tone slider (Exposure, Recovery, Fill Light, and/or Blacks).
May 28, 2009
In the Histogram panel, use the fly-out menu to select either Expanded View or All Channels View. Then, click and drag in the histogram to view the Levels, Count and Percentile of the range selected.