When you render the Material ID channel, objects which use this material will have the specified color. Previously, the colors were randomly assigned at render-time.
A related hint too -- if you click the help button in the top of the Scene Manager, you'll see a new docking window (or regular modeless window, depending on
Options > Dockable Help Viewer); it has context-sensitive help which can be shown by hovering the cursor over various places in the plugin UI. When there's help available, the cursor will turn into a question-mark; when it does, right- or left-clicking will bring up the help for the item under the mouse. Pretty much everything in the whole plugin has some entry in there, sometimes with quite a bit of detail.
And I'm not saying RTFM or anything here -- I am just guessing, from the question, that you don't know about this yet.
