From an AOM:
YELLOW ELEC PUMP pb sw (springloaded)
ON : The electric pump is ON.
If the electrical power supply is removed, the pump will remain off when
electrical power is applied again.
Off : The pump is off.
It comes on automatically when a crewman sets the lever of the cargo door
manual selector valve to OPEN or CLOSE.
This inhibits the operation of other yellow system functions (except alternate
braking and engine 2 reverse).
BLUE ELEC PUMP pb
AUTO : If AC power is available, the electric pump operates :
– In flight
– On the ground, if one engine is running or if the crew has pressed the
BLUE PUMP OVRD pushbutton on the maintenance panel.
OFF : The pump is de-energized.
The words "in flight" appear beside the Blue Elec Pump, but not beside the Yellow Elec Pump.
It's still unclear however, whether one could turn on the Yellow pump in an emergency. I know there is a pump termperature issue and that an A330 burned at Toulouse as a result of a yellow pump being left on overnight (or something like that) and I see noting in Limitations ATA29 - Hydraulics on not operating the Yellow Pump.