There is no purple line.
The colors of lines are red, blue, darkgreen, brown and orange.
The blue line and the orange line represent voltage relative to the ground.
The darkgreen line and then brown line represent voltage relative to the cathode of the tube.
The definition of the plate voltage is the voltage between the plate and the cathode.
So, true load lines are the darkgreen line (for V1) and the brown line (for V2).
The reason that the darkgreen line is different from the brown line is that the input voltage is applied only to the grid of V1.