I always find it interesting when people cannot objectively review a series or a movie because of previous bias or rumors they read somewhere. When "2Moons" ended everyone fell in love with Kimmon and Copter and some of the other actors in that series. Then, when it was announced that a sequel was coming but with a completely new cast, people went crazy because they couldn't understand how the company was managing its actors. But, "2Moons2" aired, with all new actors and then people fell in love with Earth (playing Wayo) and the steamy scenes between Pavel and Dome (playing Forth and Beam) were also incredibly popular (just go to Youtube and you'll find A LOT of reaction videos on that). So, the world didn't end because the cast of "2Moons" was completely replaced, right?

All this preamble is to say a similar thing about Color Rush S1 and S2... you simply cannot compare both just because the character of Go Yoo-han is not present in season 2, and here is why...

1. The acting of Yoo-jun (playing Choi Yeon-woo) remains as one of the solid points of both seasons. In the first season he deals with the fact that he is a mono, his mother is also a mono but is missing, and he finds new friends and also finds Yoo-han who is also his probe. He experiences the rush of being able to see colors and also the disappointment of losing that ability after the decoloring effect begins. In season 2, he goes from the uncertainty of his mother still being missing, to the dread of witnessing Yoo-han's kidnapping, to the potential of uncovering all the illegal activities the Gray Scale/Woksun/Nerve Candy people (led by a mono that goes by the nickname of "Monowife"), to dealing with his new feelings towards Kim Se-hyun, to finally finding out his mother did die but being rather calm about it.
While I think some of his facial expressions still need a bit of work, his eyes do manage to express a lot of the feelings Yeon-woo goes though in both seasons while showing the calm but decided character that Yeon-woo needs to be.

2. The acting of VIXX's Han Sang-hyuk (playing Kim Se-hyun) was also a nice surprise as he never doubted or was hesitant about acting in a BL story (according to an interview he gave) and showed a very strong character that goes from befriending Yeon-woo, to trying to help his sister improve her condition as a mono and not be bullied by others, to mourning her death, to seeking revenge while protecting Yeon-woo, to confessing his feelings for him despite knowing he might never get a chance if Yoo-han returns. Sang-hyuk was able to mix some serious tones in his acting with some comedic moments, like in ep. 8 when he daydreams he is kissing Yeon-woo.
The chemistry between him and Yoo-jun was quite strong as they acted like a married couple in some parts (like when Se-hyun moves in with Yoo-jun) or like Sherlock Holmes & Watson when discovering the truth about the Lapis Lazuli tea, the Lapizata pill, and the illegal probe surgery or like Batman & Robin in the action scenes in the final episode.

3. The storyline had a bit more action. Season 1 was a slow-burner in many things because we needed time to process the mono-probe world, the introduction of each character, Yeon-woo's mother being a mono and also missing, Yeon-woo's aunt being a reporter trying to uncover the truth and find her sister, and then also go through the color rushes that Yoo-han made Yeon-woo feel along with all their interactions.
But, season 2 had nothing of that as we already knew the mono-probe situation and Yeon-woo's familial situation. A bit of a complain is that a lot of the action happened way too fast and we jumped from scene to scene without proper time to digest what just happened. Further, several things like a) Yoo-han's kidnapping, b) the introduction of Se-hyun as "someone who wanted to get close to Yeon-woo but couldn't because he was always glued to Yoo-han", c) Se-hyun's sister life as a mono and her part in the secret society, d) the secret society of monos trying to see colors through drugs and illegal surgeries, and e) Se-hyun's being a mono himself but being able to deal with it in secret, were not completely explained/introduced and then not fully developed... so in most parts, we need to either remember things that happened or were said in season 1 or figure out by ourselves how the characters get to the point we see in that particular scene. And also, do they never go to school? Not much happens inside their classroom this time and the secondary roles of Yeon-woo's friends are relegated to a few scenes here and there.

The only truth about this series is that Hur Hyun-jun (the actor who played Go Yoo-han in S1) was not present in S2 and honestly, I'm not sure that his absence made much of a difference. Sure, we heard the name "Go Yoo-han" many times during season 2 because of him being kidnapped and Yeon-woo's wish to find him... but at no point I felt like, "geez, it would be great to have Hyun-jun acting here". There are many rumors about Hyun-jun's exclusion from season 2... rumors about him not being happy about his first major role happening in a BL story, rumors about the production crew/staff being unhappy about his "lack of professionalism", rumors about the production company being unhappy about his "lack of acting skills", and rumors that this was all planned along because of the storyline/plot that season 2 was going to have. Whatever the real reason is, the only real fact is that Hyun-jun was not part of season 2 but comparing both seasons or how Hyun-jun would have acted instead of Sang-hyuk ends up being a useless and ridiculous exercise because, you simply cannot compare.

The storyline (especially the after credits scene in episode 8) sets up the series for a third season with Monowife going into Yoo-han's hospital room and telling him to wake up as Yeon-woo "needs him" and him opening his eyes. Could we have a third season with Yoo-han coming back and now having a love triangle if Yeon-woo and Se-hyun remain in the series as well? Could we have a third season that explores Monowife's connection with Yoo-han? Could we have a third season with a whole new plot twist?
Who knows! But it sure is more exciting to think about all these options than waste time lamenting the absence of Hyun-jun or complaining about what this season could have been like if he acted in it, don't you think?