(Note: although it may work in 3.0, the library it depends on will be deprecated in future versions. I hope to get this fixed - any community help is appreciated.)

This addon extends functionality of Blender's color picker.  It can extract the max, min, mean, and median RGB values of a 3x3 or 5x5 square, or a selected rectangle.  It can be done on any area of the Blender UI, including on images and movie clips.

3x3, 5x5, NxN Color Picker

Color pick anywhere in the Blender window. Based on the button you press, the operator will either extract a 3x3, 5x5, or a custom size square of pixels and update the panels with the square's max, min, mean, and median. Left mouse click to save the colors and close. Press the right mouse button or the escape button to cancel the changes.

Rectangle "Screen Grab" Color Picker

Left click to pin one "corner" of the rectangle. Afterwards, right click to pin the opposite "corner" of the rectangle (you can go in any direction). After pinning these corners, the operator will extract the rectangle's max, min, mean, and mean. Press escape (or right click if you haven't left clicked yet) to cancel.

Note: these values are gamma corrected, so they reflect the values you see on the screen, not the true values in Blender before the colorspace conversion.

Limitations

There is currently no UI response to the screen grab color picker.  Due to limitations with bgl, the best way to implement this is yet to be determined.

Updates

February 2021

Works up to 2.91.2, corrected max and min colors

January 2022

Works up to 2.93, added a custom tile size option