It becomes a lot more fun if the resulting color is also polarized. That'd increase the "bandwidth" plenty, and also add in the possibility of being misinterpreted.
Is it essential that it is a syllabic language? If they communicate from eye colour alone, their language might well be completely alien. So perhaps different tones could convey different situations, emotions, ideas, etc; like a pictographic language? Maybe purple(ish) = no/not/bad/HOW DARE YOU POKE MY TENTACLE YOU SON OF A CUTTLEFISH, and blue(ish with green and just a dash of red) = sea + berLUDDD = dinner is on its way
Forgive me if I've ignored something you said, as I'm really not awake right now. I just get these strange ideas. Good luck.
Important principle to remember: things that are common will take a short time to say, things that are uncommon will take a long time to say. So a human observing them might note a particular twitch/color/flash that indicates a transition between concepts, a pronoun-equivalent (like 'I', or something else that would be referred to often).

Meh, Eldritch Abominations that communicate in the visual spectrum aren't likely to be all that eldritch to begin with.