Comment 2 for bug 644993

Here's another idea along these lines:

When a tag is a sub-tag of another tag that has a color set for it,
provide the user with a selection of unused shades of the parent color
to use.

In fact, this is how I set up my own color system manually. I pick
shades of blue for different x.org packages I work on, shades of yellow
for projects related to my work on launchpad, shades of grey for
household chores, etc.