Another nice tool! A couple recommendations:
-Offer an Always on Top window option. This guarantees visibility of the tool even when the user needs to click/move/show/hide other windows.
-Register F1-F3 as Global Hotkeys. Currently Pixel Picker must have focus for the hotkeys to function. If it does not have focus, the user must go back to the tool, click it, then move back to the screen location of the desired pixel to select it. More concerning is the issue that F1-F3 will invoke any built-in function of the currently focused window tied to those keys. If the user is trying to select a color and does not realize PP has lost focus, this could easily cause unexpected/undesired effects.