I use a nice little app on my Windows platform, "Color Schemer ColorPix" from
http://www.colorschemer.com/ (Not to be confused with one with the same "ColorPix" name from WindowsUtilities.com).
It is a single 579K .exe file, that when you start, shows a small window that changes as you roll the mouse anywhere on the screen. Its magnifier enlarges the area around the mouse, and it lists the RGB and HEX numbers for the color of the pixel under the mouse pointer. I have been using it extensively to figure out just how Threads styles are set.
Here is a view of its window: