By default the shortcut for the color sampler tool while painting in Krita is CTRL. I’ve been using this for years but I decided to change it to ALT recently because I use other programs that all use the ALT shortcut. It makes things simpler if similar programs all use the same shortcuts. The less I have to think to do regular actions the better.
This can be changed In the settings, canvas input panel. Change the setting for Sample Foreground Color from Merged Image to Alt + Left Button.