Open a command prompt, and browse to the DesignCAD folder.

Go to Files\IMSI\DesignCAD 3D Max 24, then issue the following commands: 

Dcad17 /reset Dcad17 /clear Dcad17 /new Restart DesignCAD

Restart DesignCAD and it should be back to normal.

You may have to close and restart once more for the color toolbar to appear.