The histogram panel in Lightroom is a jewel that you should keep your eye on at all times while developing your photo (hence why Adobe made it possible to “dock” it in place while the other modules in the Develop panel will scroll up and down). Apart from the standard graph of colour values that we’ve come to expect and the basic shot metadata (ISO, focal length, aperture, & shutter speed) it contains a couple handy tools that are invaluable when adjusting your images: the ability to view where highlight and shadow clipping occurs in your image (if at all).

The previews can be toggled on or off by simply clicking on the icons in the upper-left and upper-right corners of the histogram, respectively (alternately, mousing over the icons will momentarily enable the appropriate preview). When enabled, the shadow clipping preview will overlay blue over all regions of your image where no colour is present (ie: completely black). Similarly, the highlight clipping preview will overlay red over all regions of your image where the colour has been blown out to completely white (you’ll notice that occasionally the icons will change to one of the colours displayed in the histogram: this indicates that only the matching colour/colours have been clipped).

An example image with highlight & shadow clipping previews turned on is below:

Turning the previews on before making any adjustments that might result in clipping occuring (ex: exposure, blacks, brightness, colour luminance, etc.) is never a bad idea. This doesn’t mean that you should never have 100% black or 100% white in your images; just be sure that those states are present in an image because you chose to include them, not because of an ill begotten side-effect from another adjustment.