I get the theory of wanting to use CMY inks. I just think it’s weird and a lot of extra work for such a narrow goal. There’s not really any wrong way to use your pens however you want, provided you’re aware of the risks and know that you may be spending more time than it’s really worth.
There’s minerals and other things in tap water. These things will cause issues and damage.
Though, the Lamy Safari is a cheap enough pen to not worry about buying a new one after causing damage.
However, there are other considerations for not using printer ink. Printer ink is designed to be used on specific paper, while fountain pen ink is designed to be more versatile.
The colors for fountain pen ink are also more than the base color. There’s the shading which can be a different color from the base The sheen, which can make the ink shine a different color in different lighting. It’s designed to flow properly through the feed with an appropriate level of viscosity. There are a lot of factors and testing that goes into fountain pen ink. You can read more about it here: www.jetpens.com/blog/…/968
This is just pure chaos. Why would you use printer ink in a fountain pen? I cannot fathom any valid reason besides just wanting to do something batshit insane.