QTR-Quadtone RIP

Hi,
I'm printing with only black and photo black turned on , on my Epson
R800. Despite this it looks that my printer is still using CYAN,
because looking at the printer monitor it suddenly gave me the message
that the CYAN cartridge is almost empty ! 
So, it looks to my that CYAN is being used although not turned on in QTR.
Can anyone explain this to me?

Thanks,
Herman