Canon Pixma iP90 User Manual page 126

Ink Saving Settings
You can be set as the mode which saves black ink. Moreover, when a black
ink tank is empty and a new tank is not immediately available, you can
change to the mode which can be printed only in color ink as an emergency
measure.
(1) Save Black Ink
Print with less ink consumption.
(2) Use Composite Black
Black is printed in color ink, without using black ink. This option is
utilize when there is no black ink.