Updated to show more current screenshot and clarified the question (Which tools? What are the steps?)
I'm learning QGIS (1.8 "Lisboa" for Mac).
On the QGIS Layer labeling Settings window there is a section called "Data defined settings":
I believe the user's guide briefly describes its purpose:
The advanced entries inside the Label tab allow you control the appearance of the labels using attributes stored in the layer. The entries beginning with Data defined allow you to set all the parameters for the labels using fields in the layer.
But there are no instructions for using the Data defined entries and the Help file doesn't mention them at all.
For example, I want the label color to be white for the darkest polygon colors in a choropleth map. Will the Data defined settings help me do this in QGIS? If so, how? (Cartographica, for example, allows a user to easily set individual label colors for each category in a choropleth scheme. I was expecting a similar function in QGIS.)
One commenter suggested using the "Expression based label" tool. I don't know how to access the tool and there don't appear to be instructions in the user's guide. It isn't clear to me whether "Expression based labeling" supersedes "Data defined settings" or is meant to be used in conjunction with them.
I'm in need of clear, step-by-step instructions.