Holding Alt in combination with the color picker to "Flash highlight" the selected colour across the whole display.
(Very handy for helping visualize color shapes when working with or tracing raster images as this function would work on a display level and would be basically a visual aid)

i.e. holding alt and then moving the color picker over the display would (as well as what it usually does) "flash highlight" (flash once this whole color (with a high contrast colour) across the whole display! (i.e on a pixel level)

So when Alt was depressed this "Flash highlight" would occur when ever a new color was encountered underneath the mouse pointer...
This would allow the user to quickly identify all regions of this color across the whole screen.

(You could also have options for this: ignore locked layers and highlight vector objects only....)
these 2 options would change the way this function works....ie normally this function would work on a display level ignoring weather a color was the result of an underlying raster image or an overlaid vector object and simply working on a display level (i.e functioning as only a visual aid) but with the "highlight vector objects only" option it would then function to help quickly "flash highlight" vector objects of a specific color. This second option could be triggered by holding shift instead of Alt....
Adding very easy and fast access to these new functions onto the existing color picker tool.

https://blueprints.launchpad.net/inkscape/+spec/display-color-highlight