How to know the color hexadecimal value of the picture or screen display

If you see the specific color of one picture or displayed in the computer screen,  and  you want to copy that color to your project, so your project have the same color. How to do that? 
You have to know the hexa value  of that color. 
To know the hexadecimal value of the picture color we need the tool called WhatColor, download here,  stand alone file small size, about 200KB for windows.
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