The plot is really interesting, but poorly executed.
The "color rush" and its effects are well done, don't get me wrong. The idea that the awe-inducing feeling of being able to see colors for the first time in your life can cause confusion and a phisical reaction is on point, as it is the idea that the fear and desperation of having to go back in a gray world when you lose sight of your Probe can bring Monos to extreme gestures.

Too bad that's where the good work ends.
This same aspect is not further explored if not just enough for the story to go ahead, and the rest of the sub-plots elements are barely aknowledged: Yeon Woo's flashes of (possibly?) someone drownig are too on the nose and yet never addressed; the spirit possessing Min Jae has no purpose; the disappearence of the mother is not really explored either, let alone resolved.

I suppose they were sure to be granted a second season, but even if that's the case, this is NOT how you build a first season. If you don't have the time to resolve all the sub-plots (which I completely understand ) pick only one or two and focus on them, maybe leaving small hints for the ones you will focus on in the second season.

For example let's say in the first season we focus on the color rush and the way Monos become obsessive. Maybe making them seen as a threat to society, as if they're all psychopaths because of their genes. In 120 mins we would have enough time to explore these two things enough. They're linked by what we are experiencing together with Yeon Woo, so it's easy to expand both concepts.

At the same time, in preparation for the second season make Min Jae say weird things every now and then but without explaining why, so that we are left wondering wtf was that about, and the second season can expand on that, giving the pg an interesting backstory.
Same goes for the mistery of the mother. We can watch the aunt go for long travels and have little hints suggesting it's not just work-related but she's up to something, and that all the attacts perpetrated by Monos may be linked to what happened to Yeon Woo's mother. Nothing too on the nose, just little things here and there. Let the viewers put the pieces together. I swear they are smart enough to do that, and it would make them appreciate the whole drama a lot more.

(Also the main couple has zero chemistry, I'm sorry. They feel very awkward whenever there's a more "touchy" scene. Let the poor boys spend some time together or something, I'm begging you!)

The love story between the two main also feels very rushed. This trope is very common in fanfictions, but it works there for two reasons:
1. Usually these fanfictions are not featuring original characters, so even if they are "AUs" in which the pgs don't know each other, the reader already knows and likes the couple and doesn't feel anything missing.
2. It is well known that this trope is under the "soulmates au" umbrella, meaning that the two main characters, being soulmates, immediately feel a strong connection and their feelings develop faster than normal. This is undersood and accepted by the readers.
This drama does not have these 2 points on its side. It could work with the second one, but if that's what it was going for it does a terrible job explaining it to us.

Lastly, but this is just a personal preference, I would have liked to see the world more through Yeon Woo's eyes. He sees everything in scales of gray until he meets Yoo Han, so why not make the bold choice of letting the viewer also see everything gray up until he has the first color rush? and again every time we see something in the pov of Yeon Woo? I think it would have made it all the more impactful. But as I said, personal preference.