Creating a coloured flag that changes colour over time

Depending on what exactly you're after you may be able to get away with conditional formatting.
For instance, if you have a date you open each task in in, say cell a1, and the task in B1, you could use conditional formating to mark items 3 days old or older by going to conditional formatting and selecting formula and using the following:
=TODAY()-$a$1>=3
and you could make the font red, for example. Conditional formatting would allow you to use 3 different markers.

Sorry if I'm way off base with what you were after.

good luck.

IML's suggestion will work, but just as a caution, my accounting experience has taught me that relying on numbers is much more dependable than relying on colors. You can perform more functions and calculations against numbers than you can colors.

I'd strongly suggest that you create a column that evals the difference between dates and then format that column to highlight over 30, 60, 90.

Kevin