Officejet pro 8720 can't print with black only when colored cartridges are empty

[aerealty]

Is there a way to get around having to buy more colored cartridges to be able to print in just black ink? My Officejet Pro 8720 won't allow me to print with just black ink since the colored cartridges are empty. 

[Bob_Headrick]

Sorry, your HP Officejet Pro 8720 will require ink in all the colors to prevent printhead damage.

HP and other printer manufacturers have different printer technologies for different needs.  For inkjet printers:

• Some have printheads built into the ink cartridges.  These printers can typically print with one or more of the colors completely empty, or even with color or black (but not both) cartridges removed. The Officejet 8040a is an example.
• Other printers have separate ink supplies and replaceable printheads.  An example would be the Officejet Pro 8720.  The printhead in these printers can be replaced if they are damaged by running the printhead with colors out.  Some of these printers will allow printing with a color out, others will not.  If the printer is run without ink in one or more colors the printer may be damaged, but the user may be able to recover without having to send the printer for service. 
• Other printers have permanent printheads.  To run these without some ink in all the colors would risk causing damage to the printhead due to clogs, air ingested in the printhead or burned out printhead firing resistors. The Officejet 6700 is an example of this type of printer. For printers with fixed printheads this could require service to get the printer to print properly again when the ink is finally replaced.  

The document here describes how ink is used.  While the document is written for HP Inkjet printers the same principles apply to inkjet printers from other manufacturers.