Currently the Color Sampler tool always use the cursor indicating picking color from a layer ( samplerLayerForegroundCursor() ). It would be better to make it alternate between samplerLayerForegroundCursor() and samplerImageForegroundCursor() according to the mode selected in the tool options (respectively "Sample Current Layer" and "Sample All Visible Layers"). This would make it more consistent with the color-picker shortcuts of the paint tools (ctrl and ctrl+alt) which already use those different cursors, and give a visual clue of the selected option.
*** This bug has been marked as a duplicate of bug 419715 ***