I assume, that you want start a graphical tool from the command line, and that you want to do this within a running X session, using a window manager like kde or gnome.
Can you please post the error message?
sudo is a tool to allow users running programs with other users rights. E.g., to allow a non privileged user to run the "passwd" program for root. Or something like this.
Perhaps the following is your problem:
If you want to run a graphical program as user b, but startet the X-session as user a, the X-server denies user b any access. For security.
The corresponding error message looks like:
Error: Can't open display: :0.0
User a can allow other users to use his X-Session with