My 10/10 rating is a subjective score on my own enjoyment. Because I think this is my first review, my objective scale is on a Gaussian curve (plurality of manga rated 5/10 and the number goes down toward 1/10 and 10/10), on which I would probably give Rainbow and Black a 7/10. The manga itself does what it has to do as a solid and easy read beyond many manga.
Kuro does not exactly "[feel] like she's living in black and white," but more so comes to a realization of the consequences of her treating things in life, be it friendship or manners, as black and white. She stumbles upon Niji in a cage on the side of the road after being fired from work. In a logical decision based on simple human empathy, she decides to bring the creature back home before she can hand it off to a more suitable caretaker. She then begins her relationship with a more colorful side of life through Niji, as most pet owners could imagine. This beginning premise of adopting an abandoned animal sets up an easy foray into a relatively believable setting. One thing, though, is man oh man does the ending section to the first chapter provide an incentive to continue reading on.
Besides Kuro's rarer personality and despite the fantastical nature of the bird-thing (which you can clearly gather from the cover), the story is quite grounded in reality. Its use of contemporary devices like the increased accessibility to experts in specific fields or the fickle and sudden trends of social media expand the plot quite naturally. Most of the development is contingent on Kuro's changing outlook on life from learning about and responding to Niji's capabilities (which are flexible but easy to expound for the author because Niji's species is fictional) as well as the major results of the aforementioned plot devices. Niji's antics and conduct are almost always well-executed comedic happenings. Although, I admit I would partially discount a bit of my own comfort at times due to how Niji's perspective on who Kuro is in his own life influences some of his behaviors. The lack of an explicit antagonist drives home a pure slice of life narrative. Overall, the plot and development provides a comfy atmosphere where you don't have to worry much about plot holes and the like. You just have a calculated glimpse into Kuro's life in-home with Niji and outside with side characters.
There's not too much to strongly laud in terms of the art of the chapters. I don't know too much about manga artistry, but it seems hand-drawn by the generally imperfect lines with screentones for parts like the happy mice's feathers' textures. Parts like Niji's cage, heavier with details, are of course a bit cluttered, while the remainder of the panels are just detailed enough to my taste. The colored volume covers and inside covers are quite nice, and my favorite parts of reading this manga as a whole were those for volume 2 (they hit the right spots in my personal aesthetic). I'd hazard it purposely highlights the potential of how much of an elevated visual attractiveness the series would gain if the remainder of the pages were also colored, just like the juxtaposition of Niji and Kuro.
The main criticism I'd give is the jarring transition between the second-to-last and last chapter. It wasn't so bad as the whiplash from rushed endings of long-form manga like MÄR due to being axed or author's choice; however, as I started reading the last chapter, I had to check whether or not it was the last one, otherwise such a jump in the plot wouldn't make sense.
All in all, I'm glad I just so happened to find this manga. It was published in Comic Rex, a shounen mag, which gave it its listed genre, but doesn't feel like it's targeting a specific demographic. I'd recommend it to those looking for a slice-of-life with a twist beyond humans being humans, like Tokedase! Mizore-chan, or just wanting something cute to read for a bit.
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