The Color Picker Tool in GIMP serves the single purpose of selecting colors from an image, ensuring a perfect color match that would be very difficult to achieve manually mixing colors.
The Color Picker Tool is used by simply clicking on an image and the tool will use the color value that it selects in different ways, depending on how the Tool Options have been set.
Although the Color Picker Tool serves a relatively straight forward purpose, there are several Tool Options that affect how the Color Picker Tool works.
Sample Average
If this isn't checked, the Color Picker Tool will select the color of the single pixel that it is clicked on. When this option is selected, the number of pixels sampled can be adjusted by using the Radius slider. When set to one, all pixels within one pixel of the clicked pixel will be included in the sample, meaning that the selected color will be the average of nine pixels in total.
Sample Merged
This option only comes into play when an image contains more than one layer. If selected, the resulting color of all visible layers will be selected. When not selected, only the color of the active layer will be selected, regardless of what the visual appearance of the color may be.
Pick Mode
By default, the Color Picker Tool sets the foreground color to the selected color. However there are other options, allowing you to set the background color, add the color to the palette or just pick the color.
Use Info Window
When Use info window is activated, whenever the Color Picker Tool is used, a dialog displays the color values of the selected color. This can be useful if you need to make a note of color values for use in another application and if the Pick only tool option is selected, this must be activated in order to read the selected color values.
The Color Picker Tool is one of GIMP's utility tools that we all take for granted, but it is very easy to overlook the options available for adjusting how the tool operates and how its color measurements are then used.