That Time I Got Reincarnated as a Slime: Tensei shitara Slime datta Ken

[Munsu]

You didn't miss anything. This is the first time we've seen these guys. You had the same reaction I did when reading the manga. They're not set up, properly, which is odd for this series, and come out of nowhere.

Nope, read my previous post. The Archdemon/Primordial at least has appeared a few times. In episode 23, he mentions that he committed a mistake when Rimuru finally summoned a demon (for Ramiris), he intented to be summoned himself but the other demon got in his way (his family as he called it). Later in episode 24 we get a flashback episode of Shizu and she meets this demon. So that at the very least shows us a connection between Rimuru and this demon via Shizu. Not sure if that's in part why he wants to serve him or not.

He once again appears within the last 3 episodes of the OVAs. I think he also appears in one of the first episodes of this season, but that haven't bothered to confirm.

As a colorblind person, does Rimuru look any different as a Demon Slime? Is he, like, purple now or something?

He looks a darker shade of blue, but not sure if that will remain after the evolution is completed (if it isn't yet).