The Windows Store update was a little slow in being certified.
You can check which version of ArtRage you're running. From the ArtRage 'Help' menu, choose 'About ArtRage'.
The version number is in the top-left of the panel. For the latest build of ArtRage Vitae you should see 7.1.3
This has the fix for the colour sampling tool.

If you see an earlier number and you're using ArtRage Vitae, you can update from the Windows Store, or run Windows Update.