this is a very old thread, but in case someone needs to do underprint with the C711WT (for decals/stationery) it is doable.

swap cyan and white toners/drum (the whole thing). be careful the white drum has an extra pin at the bottom. compare to the cyan one and match the 2. I extracted it with pliers.

then you have to go into diagnostic mode and disable toner sensor and ID sensor. there is a guide online for C710.. look for c710 maintenance mode. basically turn the printer OFF. hold menu up, menu down and help while turning on the printer. you get maintenance mode. enter password 000000. follow the guide to disable sensors. both for toner and ID.

then you need to use a software to map the toners.

refer to this guides: https://whitetonerconversion.com/

but remember: C711WT works flipped compared to C822… if you follow the guide to a T you end up with a underprint (what OT is asking)

have a nice day