Thanks Jonathan ! 
- changing color depth of the client : no change
- download displayutil and the last try “displayutil -M” resolved the problem !
Ouf !
One strange point : the options l (similar with L) gives :
0 1x1 8
0 1x1 16
0 1x1 32
0 800x600 8
0 800x600 16
0 800x600 32
0 1024x768 8
0 1024x768 16
0 1024x768 32
0 1024x768 32
There are 2 lines at 32 bits…:mrgreen:
And you know what ?
When i drag the displays menu in the menu bar, i find 2 lines at millions of color.
If i select the bottom one, it’s ok (as you can see above); if i select the other, i get the bad screen !
My mac mini was in thousands of colors. It also have the same 2 lines.
When i select the “mid 32 bit”, same problem !
Come back to the 10.4.10 (spare disk) : same problem (2 lines).
Conclusion : Not a pb of os version but a xserve problem, i presume.
A little very useful utility " a garder dans sa besace" !