by Julieanne Kost
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.