You are right that paint is kind of its own thing and doesn’t really fit into the RGB or CMYK systems

But I would say it’s overall still subtractive. The paint and whatever you’re painting on isn’t giving off any light on its own, its just reflecting whatever ambient light there is (which is usually more or less white) and subtracting from that.

You could maybe argue that it’s more replacive (is that a word?) than additive or subtractive. It just kind of is what it is. It’s just replacing the substrate’s reflectivity with its own since it’s opaque like you said.

And when you mix paints it tends more towards that grey-brown because like you said it’s not layered, it’s more that each pigment is right there on the surface next to each other reflecting and absorbing their part of white light.

So if you mixed cyan and magenta paints together, instead of light passing through layers of cyan and magenta until all the red and green are filtered out so that only blue light reaches the white paper and is bounced back to your eye, you’d have cyan piments reflecting blue and green, mixed in right next to magenta pigments reflecting red and blue. So both are reflecting blue and the resulting color will probably look blue-ish, but the cyan is reflecting some green, and the magenta some red, so that pulls the color more towards grey (somewhere between white and black, it cant really get down to true black or true white because some light is always going to be absorbed and some reflected)