User Preferences > System
Change “Color Picker Type” to "Square(SV+H)
Try Sampling colors in the properties panel.
My observation is that the color one samples is not the color one gets.
It appears as if the ‘V’ (value) information is not considered correctly.
Peculiarity enough, it does work correctly in ‘Texture Paint’ and ‘Image Editor’ panels.
I would like secondary confirmation of this oddity before reporting as a bug.
I have not gone through the entire interface to see where it works and where it fails.
I’m pretty sure It is, you pick colours in linear and they are converted to sRGB when rendered or shown in interface. However if you paste a hex code it is interpreted as sRGB and converted to linear HSV.
With colour management/gamma correction off (ie none display device) the colour shows as picked in the picker.
A simple component to select color in the same way you select color in Adobe … Custom skin and using flat mode to display the color picker in a custom widget.