It is impossible to for a second party tell a first party that they have been unsuccessful in imagining something.

Looking at the last panel, I can say with certainty, that dude failed at the task.

It is inherently a counting problem because of how sight and color recognition functions.

It's, again, no question of sight and color recognition but about imagination.

You're still looking that the comic from a very wrong angle and say "it makes no sense". Well, from my angle, it does.

It's a thought experiment, reminds me of zen Buddhist koans. "What is the sound of one clapping hand?" or "What did your face look like before your parents were born?" don't have an answer. You can tell me you know the answer and I can't proof you wrong but that's not the point. It's about making people think. "Imagine a color you never saw" is the same. You can tell me you made it and maybe that would mean enlightenment for you but it's beside the point. It's a thought experiment obviously meant to have no answer (again, look at the last panel). The more you tell me that makes no sense and there is no answer, you're proofing my point. The comic makes it explicit that there is no answer. You impose a very different meaning onto it that doesn't lead to anything and say "the comic doesn't lead to anything".