P600 CISS Question

I have recently purchased the CISS for my Epson P600 but have not installed yet, as I am using the OEM inks that came with the printer. However, one of the carts is looks as though it is getting very low while the others are all half full or more. Can I selectively install the CISS one color at a time or do I need to install all when my first OEM cart is empty?