I'm a big fan of black ABS. I like my parts heat-resistant, and black usually goes with everything. Most of my clients are happy to print in black, so I generally tend to go through several spools of black at a time. Ours sales also point to the overwhelming popularity of black.
That said, there are times when I have to switch filament. A recent customer print specified clear PLA, which is about as opposite from black ABS as you can get. I wanted to dial in the machine and verify everything was working with PLA, so I just switched filament and flushed the nozzle for about 2 minutes, then started the print.
Unsuprisingly, the clear PLA was contaminated with black ABS, causing the part to be slightly darker than the glass-clear color that you'd expect. What I didn't expect was that it would stay that way for as high into the print as it did (in retrospect, the part has a pretty small cross-section, so that would explain why). I decided to huff-and-puff-and-blow plenty of air into the side of the extruder, where I was sure my blowing would knock loose any remaining ABS dust loitering around in the feedpath.
The result? You can see the dark-grey band just slightly above the half-way point of the piece before I stopped the print. The extrusion below that is clearly dirty from black ABS contamination. The extrusion above is nice and clear as expected. (The difference is more noticeable in real life than in the picture.)
I should get some compressed air and used that at future filament changes...