You can preview clipping in the highlight and shadows while working on various tools in Capture One that alter the exposure. The warnings are an essential aid in determining when image detail is lost during adjustment.
The feature is not restricted to working with the Levels tool alone, but it is particularly useful when manually adjusting the shadow and highlight sliders to the clipping points. Clipped points are either pure black or pure white and lack any image detail.
The Exposure Warning indicator is located in the right-hand group of the main toolbar. The preview displays a red-colored mask in the highlight areas, warning that at least one color channel (red, green or blue) is clipping and image detail may be lost as a result. Check the histogram in the Levels tool to see which channel is clipping - it will spike against the wall.
The default setting for the shadow warning is disabled. However, this can be enabled from the application preferences (Capture One/Edit (macOS/Windows) -> Preferences… -> Exposure tab -> Exposure Warning -> Enable Shadow Warning). When enabled, the clipped shadow areas are displayed in blue.
The clipping thresholds and the color for each of the masks can also be modified from the application preferences. Click on the color window and select from the swatch.
Note that the default highlight clipping threshold is below 255 (pure white) at 250 to allow a small number of pixels to clip. In most cases, the sliders can be left to the defaults.