The answer is Because the resting plasma membrane is more permeable to potassium
Plasma membrane is more permeable to potassium(kalium) than sodium(natrium) which makes it easier to move between intracellular fluid and extracellular fluid. But the concentration of the ion still regulated strictly by the cell using pump and transporter in their membrane. The pump is doing active movement(against gradient of concentration) and need ATP to do so.
Texture and color usually
