filling contour without fading

Hi there,
I am plotting 3D data using pm3d:
   set pm3d map
   set contour
   set cntrparam levels auto 10
   unset surface
   unset clabel
   set palette model RGB  defined (0 "blue", 100 "green", 110
"yellow", 200 "red")
   set style line 1 lt -1 lw 0
   splot "out2" w l ls 1

and I get nice representation of my data.
Now I would like to have an exact border for the colours, without one
fading into another. I have been googling around, look to previous
posts but nothing there.
I am using 4.2.3. Is there any trick for that?

Thanks in advance for your help

Regards

xspirix