> If someone here can help me to find a way to delimit the band
> to start at the end of the left picture and stop at the
>beginning of the right picture. 

In tde/main/tdelibs/doc/common you can find the basic pictures to start with. From bottom-right.png you can try to adjust the color to match the new schema.
And in tdebase/kcontrol/kcontrol/about the file top-right-kcontrol.png is the picture displayed at the top. We can overwrite that if necessary.

Cheers
  Michele