I don't think it is possible. I tried to look into all the properties of a parallelplot figure (and more in general of a plot figure) using the commands:
%use only after plotting
inspect(gca) %for axis proeperties
inspect(gcf) %for figure properties
but coulnd't find any that changes the plot area background.
Closest i got was to change the figure background with
set(gcf, 'color', [1 1 0]) %yellow
but the plotting area is still white.
