Empty cartridges for the P600 need to initially primed before first use. Thereafter priming is not required when refilling.

Refilling the cartridge is simply removing the rubber plug, placing ink inside, replacing the plug and then reinsertion. With the P600, refilling can be performed whilst the cartridge remains inside the printer. This removes wear and tear of the seals and extends cartridge life.

When the Ready2Go kits are purchased, priming is already performed for you.
PRIMING INSTRUCTIONS PDF
When the printer has run the chip down to EMPTY, you will encounter a cartridge not recognized warning. To reset the chip to full, you simply remove the cartridge from the printer and then replace it.  Doing this prior to the warning will NOT reset the chip to full. The printer programming in conjunction with the chip logic is in full control of this.
You should always make sure that the cartridge never runs out of ink and that the ink in the cartridge damper is not used up. This is extremely important on the R3000 & P600. If the ink inside the cartridge is totally used up, you will get air gaps inside the tubes that lead to the printhead that can be difficult and laborious to remove.  So always keep the cartridge topped off with ink whenever it is convenient. It is better to have more ink inside the cartridge than what the chip shows to prevent this from happening. If a shortcut is taken to reset the chip and the ink is not topped off, then eventually the printer will assume there is ink inside the cartridge when there is none. This will not be immediately apparent because there is substantial ink inside the tubes between the stationary cartridge and the printhead.
In the event that you do run out of ink, priming instructions are shown. If the cartridge are used properly, this procedure will never be required.
Next Important point about Switching Blacks.
Prior to a switch from each of the Blacks, the printer will check the estimated ink level in the cartridges to determine if there is enough to accomplish the switch. Each switch consumes a quantity of ink.  The printer does not want the ink to run out during the switch. A situation arises that if the to be switched to black has insufficient ink, it will not allow switching until a fuller cartridge is there. However, the only way to have the resettable chip get fuller is to use and then let it switch and use it to reset.  You will not be allowed to switch. You will be stuck.
To avoid this situation,
do not perform a switch AWAY from a black while the chip indicates a very low level. Switching back will not be possible unless: You install a cartridge with a fuller reading. Like a new OEM cartridge.  Do not run the OEM Black cartridges down to empty. In fact , remove them prior to empty or low and store them  as recovery items should you get stuck in such a situation.
INITIAL PREFILL OR PRIMING