My colour picker works if I sample within the LR image however I thought it was possible to use it to sample from another window eg colour.adobe
Is this possible
Yes it is possible.
When the Lightroom Color picker panel opens up- Click within the color panel first then hold down the left mouse button as you drag the 'dropper' cursor over any color elsewhere on the screen. When you release the L-mouse button you have selected the color under the dropper cursor.
You could have any color swatches visible beside the Lightroom window to choose from. (even a color patch as an image on the Filmstrip!)
(Discovered- it will not select the (Windows-10) Desktop, but will select color from another window of swatches)
This does not work on my iMac see reply to post
This is exactly what I do but dropper does not make selection. Tried 2 different browswers but still no luck
see image below, cursor on blue but not selected in LR even when I release mouse button
Annoying for you that it does not work.
My only personal experience is with Windows-OS on two PC systems (desktop & laptop) the Color Picker works from colors in another window or the task bar, but not from colors on the desktop background.
I have done a search to see if there might be problems specifically with Mac OS and the color picker but cannot find anything that answers your question.
If you are on Catalina then this rarely mentioned 'feature' may have been lost for this OS.
I suggest you post your answer at the Adobe Feedback site where there may be a better chance of an answer.
Hopefully other Mac users (& Adobe techs) will reply to you.
Some common suggestions for 'color' problems are- Reset the Preferences, Turn GPU Acceleration 'off' in Preferences, Check the OEM for Graphic Card updates.
A few (old) links that discuss the Color Picker-
"This is exactly what I do but dropper does not make selection. Tried 2 different browswers but still no luck
see image below, cursor on blue but not selected in LR even when I release mouse button."
I'm on Windows 10 build 1903 and the LR Classic 9.3 Split Toning eydropper sampling with left mouse button does not work with Chrome, FireFox, or Edge browsers. It does work with PS, Bridge, InDesign, and XnView, but strangely not with LR Desktop 3.3.
This is crazy!!! The more I try to select colors with the dropper, external to the LrC color picker window, the more it changes!
I can select from a color swatches image shown in a Bridge window- no problem.
I can select from color in the posted screen-shot by GregC6 above in this thread (Firefox browser) -no problem.
I cannot select from the same page at color.adobe.com (Firefox) - Strange!!!
So I am out of suggestions for GregC6.
Lightroom (Desktop v3.3) does not show a dropper for me. Only the cursor arrow can select from the Lr Color window.