How can the activation key be changed?
From the Color Doggy menu in the top left corner of the screen, select the Preferences… command to change the key that activates the Color Doggy magnifier.
Why do the measured color values sometimes deviate slightly?
When displayed on screen, color data undergoes several conversions between the different color spaces of the source, the software used, and the screen. Rounding and clipping errors can occur thus leading to deviating values.
Depending on how industry standards are interpreted and implemented by the software manufacturers, there can be, for example, deviations in color reproduction when using different browsers.
You can eliminate one source of error by setting the color profile of your screen to sRGB. Note, however, that with this setting your display may have difficulty rendering all the colors in sRGB one hundred percent.
Here is the corresponding article in Apple’s user manual:
Change your Mac display’s color profile