Pen plotters are somewhat like 3D printers. They move in X/Y space and typically have a pen lift mechanism. Attached to their arm can be any number of drawing instruments, from pens to pencils, brushes to pastels, and even drill bits to scratch glass.
Printers (we'll focus on inkjet for this article) blast tiny ink particles into paper and can seamlessly mix them to create a range of visually distinct hues and tones. In the art world, they're commonly used to make Giclée prints, which are essentially just archival, fine art prints.
Original Art vs. a Reproduction
Professional quality inkjet printers can do a fantastic job reproducing what an artist sees on screen. Artists like Zach Liberman use them to create beautiful reproductions of digitally native artworks.
The keyword here, though, is reproduction. A printer isn't capable of mixing pigments the same way an artist mixes paints on a palette. It turns out, computers aren't very good at it either. Computer scientists and artists have been attempting to replicate the physical properties of color mixing through approaches such as the theoretical Kubelka-Munk equation for some time now, with limited applicable results for digital artists and creators.
I say this not to bash the RGB model that most digital artists are familiar with, hell, I use it all of the time myself when creating art for screens. It's just, when you're going for realism in color, nothing quite beats the real thing.
Check out spectral.js for the best code-based implementation of the K-M equations that I've encountered.