Holy cow, I wrote a book!
In a presentation a few years ago,
there was a pie chart,
but not just any pie chart,
but a pie chart that appeared to be specifically
designed to convey no information whatsoever.
(The presenter didn't realize this, of course,
and probably thought it was awesomely cool.)
The pie chart consisted of five wedges, each of which was a
slightly different shade of green.
No, wait, that's not right.
Each wedge was a
gradient from a common shade of green
to a slightly different shade of green for each wedge.
Accompanying the chart was a legend that described what each
shade of green represented.
It was completely useless.
(Inspired by this story, my friend
:: Wendy :: created
her own monochromatic pie chart.)
If you're going to make a chart with a legend,
then the items labelled by the legend should be colors which
are unlikely to be confused with each other.
And for goodness' sake,
don't make them all subtly different shades of the same base color.
(For bonus points, consider
the color-blind members of your audience.
And for double bonus points,
also consider the blind members of your audience.
But that's another topic for another day.)