The node previewer just shows a snapshot of how the node is applied to the model in the preview viewport, not the color you’ve picked. So it takes lighting etc into account. (E.G how when you enable live preview on nodes with UV deformations over time etc, you can see the actual deformation in the node preview window)
If you right click the node and and select “Start previewing node” you can see where it gets it from.
I don’t think it’s intended to show true color.