You could do this via a set of macros (one for each colour), which you assign to a series of keyboard shortcuts, or a single macro that cycles through each of the possibilities assigned to a single keyboard shortcut.

Alternatively, you might consider using a combination of a single font colour and highlighting. For example, red with yellow highlight might indicate important notes, red alone for confusing content and yellow highlight alone for comments. At most, that's two mouse clicks for any combo (important notes).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]