[Frosty Crunch]

Having spent a half hour figuring out what the colors mean (which are not completely documented in the manual as far as I can tell, and are certainly not conveniently document in a single table form), I thought I'd preserve what I have learned:

[Edit: Whoops! Page 44 of the documentation, Customizing Fonts & Styles, deals with this.]

Black: An available action that does not otherwise qualify for another color

Gray: An action that is not available because it has a future start date or is on hold or its context is on hold [edited]

Red: An action that is overdue (due date and time is in the past; the default time can be set in the preferences and may be 12:00 a.m. or 11:59 p.m., depending on when you got the app, which may lead to confusion)

Orange: An action that is due soon, within 2 days by default, but this can be changed in the preferences

Purple: The next action that can be done in a sequential project, as long as it is not overdue or due soon, or the first available action in a parallel project [edited]

Blue: An available action in a single actions list, as long as the action is not overdue or due soon [corrected]

By the way, I think using Apples Font and Color palettes for changing color and fonts is misguided. Those palettes are meant for word processing like uses (and they are lame even for that). When you have less than a dozen "classes" of type to format, such as the six colors above and classes like "project names," "bucket names," "folder names," and the like, these are more analogous to styles in word processing style sheets, and the setting of colors and fonts should be accessed via a style format dialog (which could call the font and color palettes) or via the preferences. The current approach where you select a representative piece of text and it changes everywhere is confusing.