We recommend flushing color inks before installing Piezography inks, especially when using a pro model printer (with internal ink lines and dampers), to help avoid color staining in your BW prints. The reason is that it's difficult to get every last trace of color ink out of the dampers (yellow is often the worst for some reason), and if not completely flushed, will mix with the Piezography inks, causing color staining in your prints. Flushing color ink from your printer is an extra expense for the set of carts and PiezoFlush, and an extra step in the process of converting your printer to Piezography inks, but will save you time and headache of dealing with color staining, and you can reuse the flush carts many times (if you ever want/need to flush your printer to change inks or safely store it if you go away and won't use the printer for a while).
Flushing color inks prior to installing Piezography isn't required, and you can simply install Piezography inks after color without flushing in between, but you will experience some amount of color staining (worse with older dampers that have more particle build up inside), and need to do several more cleaning cycles to purge the stubborn color. Some people don't mind or notice the color staining, but other people are very sensitive to the color staining and would prefer flushing their printer than wasting precious Piezography inks by having to do multiple cleaning cycles.
Another option (for those who are willing/comfortable to work on their own printers), would be to install the set of Piezography inks and do an initial fill to purge the color and get Piezography to the print head, then replace the set of dampers with new ones and do a few more cleaning cycles to get ink flowing thru the new dampers. This would avoid getting the flush carts and be a sure way to prevent color staining from the dampers (a set of dampers costs about $200 and take about an hour to replace).
No matter which option you choose, we always suggest cleaning the printer's capping station, flushing box, wiper blade and bottom of the print head to remove color ink from those areas. These procedures are great for keeping your printer in top working condition and lasting a long time. You can follow our cleaning procedures (and see a video of several procedures preformed on a desktop model printer) here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
I hope this clarifies your options and details the pros/cons of each.
Please let me know if you have questions or there's anything further I can help you with.
Best regards and happy printing~ Dana