Measure CIELAB Colors with GIMP Sample Points

This article explains how to accurately read and monitor CIELAB (L*a*b*) color values across multiple regions of an image using GIMP's Sample Points dockable dialog. By utilizing persistent sample points, you can inspect color coordinates across different canvas areas simultaneously in real time without constantly reapplying the Color Picker tool.

Open the Sample Points Dialog

To view persistent color readouts, open the Sample Points panel:

  1. Navigate to the top menu and select Windows.
  2. Hover over Dockable Dialogs.
  3. Click Sample Points.

The dialog will open as a dockable tab, initially empty until you place points on the canvas.

Place Sample Points on the Canvas

GIMP allows you to drop up to four permanent sample points directly onto your image canvas.

  1. Ensure rulers are visible by checking View > Show Rulers (or pressing Ctrl + Shift + R).
  2. Hold down the Ctrl key (or Cmd on macOS).
  3. Click inside the horizontal or vertical ruler and drag your cursor onto the canvas.
  4. Release the mouse button over the target pixel region to drop a numbered sample point (1 through 4).
  5. Repeat this process from either ruler to place additional points on other regions you want to measure.

Switch the Readout Mode to CIELAB

By default, the Sample Points dialog displays pixel values in standard RGB formats. You must switch each point to the CIELAB color space.

  1. Look at the Sample Points dialog where your placed points are now listed.
  2. Next to each point, click the drop-down menu that currently shows Pixel or RGB.
  3. Select CIE L*a*b* from the list.

The dialog will now display the exact color values using the CIELAB model:

  • L*: Lightness (ranging from 0 to 100).
  • a*: Chromaticity on the green-red axis (negative values indicate green, positive values indicate red/magenta).
  • b*: Chromaticity on the blue-yellow axis (negative values indicate blue, positive values indicate yellow).

Manage and Reposition Points

  • Move a Point: Hold Ctrl, hover over the crosshair of an existing point on the canvas until the move cursor appears, then click and drag it to a new region.
  • Delete a Point: Hold Ctrl, click the sample point marker on the canvas, and drag it back into the horizontal or vertical ruler to remove it.
  • Sample Merged vs. Active Layer: To ensure you are measuring the final composite color of the canvas rather than a single layer, activate the Color Picker Tool from the Toolbox and ensure Sample merged is enabled in the Tool Options dock. The Sample Points dialog inherits this sampling behavior.