With this style of printer, there's around 15ml of ink in each line between the cartridge and the print head. The P900 is physically a bit smaller isn't it? If so perhaps there's a bit less, but it will still be a lot. So when you insert a new cart, you're still printing with existing ink for some time. You should not have a problem at all, even with a long pause in the printing.
The older smaller printers with small cartridges right on top of the print head were an entirely different matter. You would have the new ink come through on a print where the change occurred part-way through, but being able to see that change wasn't a common occurrence, and it was mainly visible in smooth-toned areas like the sky.