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Under the Project : Styles inspector, you'll see a bunch of canned styles that OmniPlan uses by default. If you click on the + sign below that list, you can create new styles for your own use, and you can control the task bar base color and completion color when you do so. Create such a style, then select the tasks you want to use it. Now go to the Project : Styles inspector and apply that style by pressing the function key shortcut listed next to your style. You can use the View Filter (View->Filter Tasks) to make it easy to select the tasks for a given resource. See pages 64-68 in the current PDF manual (available on the OmniPlan product page) for some more details on styling.

Perhaps I'm missing something, but it doesn't appear to be possible to specify that any tasks assigned to a given resource should get a certain styling, and so you have to assign the tasks first, then style them. Feel free to show me the error of my ways if you know a better way :-)