A problem about plotting band under pdf terminal

[FLUSH WEN‏]

Here is my main code:

set term pdf enh solid color
set style fill transparent solid 0.3 noborder
datafile = "ABC.dat"
......
plot datafile using 1:2:3 notitle w filledcurves lc "purple", \
datafile using 1:3:4 notitle w filledcurves lc "purple", \
datafile using 1:6 title "Average" w l ls 1 lw 3

And then here's the problem. Although I have set both bands' color to be "purple", they still show a different color deeper than purple where two bands overlap. And that is what I don't want. I want an additive band with one uniform color.
But if I simply change the terminal from pdf to post, I won't get an image with transparent lines. Because it seems post terminal doesn't support transparency.
So can I have a solution to this problem? Thanks a million!!