How should I clean my extruder when changing materials?
If you feel you need to clean the nozzle the best approach is a cold pull.  You can perform this procedure with many printers, however, you should seek advice from your printer manufacturer before attempting this process if you have any doubts.

With the previously filament still loaded and the tool head cold:


Begin to raise the temperature of the tool to the printing temperature of the next filament you intend to load
As it begins to warm up perform any tasks you normally perform during filament unloading (open the idler, for example). You should not be fighting against the extruder drive gear or any other part of the tool during this process!
With a fair amount of pressure begin pulling on the filament
The old filament should eventually release from inside of the tool leaving the end of the filament roughly in a nice molded shape of the inside of the tool.  Begin loading the next filament you intend to print and manually feed some filament through as the tool finishes heating


Anything left over from the previous filament including pigment or other residue should be stuck to the end of the filament.

You can perform cold pulls as many times as you wish by loading new filament, extruding a small amount, letting it completely cool and repeating the steps.