This is not really IDEX and not really multi material printing, BUT…
It is a tool changer, which swaps from printing to coloring and back. This looks kind of interesting and removes all the issues one has with filament color change, but having a bunch of sharpies and colorize the layer right after it got printed is a cool idea.
I guess the next step would be a robotic arm, painting the finished print. 
What an interesting, if odd-ball, idea. It’s too bad Sharpie pen colors don’t last all that long. I wonder if there is some alternative that would be more permanent, like maybe pens that have permanent ink-jet printer ink.
I find that sharpie colors are get sucked into the material. I normally mark my prototype with a black or silver sharpie, so I can later on see what I improved and how which print settings improved the model. Even so I mostly only write words like “V10” or “2W” (two walls) and “NS” (no support) on it. These letters get sucked into the material and spread out sideways.
Only writing on the bottom layer gives a clear writing and proper lines. In the little video on the page you can see this on the orange beak of the bird. The blue colors bleed into it at the top sections. Other sections like the white eye got bleed ins black from dot. In other models suffer from that, too.
However I see a big potential in finding a color pen type that does not bleed sideways. There could even be a pen market for colored prints and gives the name “3d pen” a really new meaning.
I wondered about that.
And, BTW, your use of the term “bleed” is correct. You don’t even need “sideways”; just say “a pen that doesn’t bleed”.
I know. That’s why I used it
I saw a different version of that on Make Anything. This one looks worth doing.