When I do multiple projects I often do the same task for each project. Since all items from Moo.do in the calendar pane are blue, two of the same tasks in two different projects look exactly the same. Usually I tag the tasks with a project abbreviation to easily distinguish them in the calendar, but I find that somewhat superfluous since the tasks already are subitems of the project.

Here are two proposals on how to do it:
Instead of to "star" items you could color them? Maybe there's a selection of 8 different colors, like in Finder on OSX. Subitems are automatically colored by the color of the parent, unless overriden.

I like the minimalistic interface of moo.do, I don't think the outline should be filled with colors. Maybe that could be a setting? To show colors in the outline or not.

Another proposals is to automatically set a color per item it's subitems according to a chosen level in the outline hierarchy. I have each projects at level 2 under "Projects" which is at level 1, so then I would prefer to have automatic colors for all my items at that level. At level 1 they could all have the same color.