[R] Use of xvfb : X11 cannot allocate additional graphics colors. Consider using colortype="pseudo.cube" or "gray"

[Gael Even]

Hi,

I use R in remote access on a Debian server.
I need X11() to create graphical things so I use xvfb-run (Virtual X server 
environnement) R to allowing X11() capabilities to R.
An other technique is to connect in shh -X to execute my R script.
With the connection ssh -X, my R script is well executed. BUT when I use Xvfb, 
some graphics are created but an error occured and  stop the script.
here the error :

" X11 cannot allocate additional graphics colors. Consider using 
colortype="pseudo.cube" or "gray" "

There is obviously a problem witth xvfb but I can't not resolve it
please help me!

Thank you

Gael Even