Oh, wow. That's pretty dumb, I hope they realise ink colours are made by combining Cyan, Yellow, Magenta and Black inks.
Okay this is a bit old but this is actually wrong! The CMYK four colour process used in inkjet printers etc. does not involve the mixing of inks at all! The colours are broken down optically into four components, and then dotted patterns of ink are applied in four layers (normally in the order Cyan, Magenta, Yellow, Key (black)). The ink drys almost instantly, different colours are caused by the overlap of different densities of dots. The reason that the CMYK process is used is because it is
subtractive; the natural colour is the white of the paper. The inks are partly transparent and each different overlapping dot of ink blocks some of the reflected wavelengths of light. A CMYK colour wheel gets darker towards the centre:

RGB, on the other hand, is additive. A perfect mix of all three should (optically) be white:

of course, when actually mixing pigments, dyes etc. this doesn't really happen. Exactly what colour you get from an equal mix of red, green and blue would depend on the product you were using...of course, most traditional paints aren't designed around the RGB model (RYB is used historically), but even so a mixture of these colours should produce a neutral (probably a mid-range grey or brown). I imagine that the reason that RGB inks have been chosen for the design has a lot to do with the way that digital sensors percieve colour; I'm not an expert on digital photography, but given that deep space probes and hubble take colour images in RGB it's presumably the most accurate way for digital sensors to percieve colour. Converting RGB colour values into CMYK or RYB values is possible of course, but maybe not accurately in a pen. People in the comments section about the pen harping on about the necessity of CMYK are pretty much idiots, or graphic designers (generally the same thing). I mix up screenprinting inks, woodblock inks, indian ink, paints etc. using additive colour theory all the time. In fact, a pen of this sort would probably need to use an opaque drawing medium in order to actually be able to replicate colours. If anything, it should use something like acrylic screenprinting inks with Red, Yellow, Blue, Black and White.
It's only a concept anyway but yeah. I'll shut up now.