Hi,
thank you very much.
The color picker on the canvas [RMB] picks always the material from the current layer. If you like to like to pick from all layers you need to hold [CTRL]+[Shift] while clicking [RMB] to pick from all layers. but keep in mind, that the preview looks maybe a bit different, depending on the metallicity and roughness value.
Also, the preview looks a bit different then what you aktually draw. So, you need to draw once to see the right blending.