NOTE: Another personal review. I'm more calm this time though. xD

Title
Really relevant and fitting. What I think is pretty cool is that the title is always the start of the story, so at the start, the canvas is blank. But at the end, Myungsoo's canvas is filled up with Rika :D That realisation just makes me go 'awww'. This is indeed the ideal title, no doubt.

Description & Foreword
That contrast is brilliant and really sums up RiSoo well. It piques the readers' curiosity and makes us wonder how these two different individuals will blend together. Short and sweet, and complements the story and title very well.

Grammar & Vocabulary
Pretty much immaculate, but there is awkward phrasing occasionally. For instance, 'his lips did still twitch' doesn't seem grammatically wrong, but the way it is worded is a little strange. Perhaps 'his lips did twitch, still' would sound more natural. Also, there is some awkward word usage like 'his eyes widened stunningly' (I get what you're trying to say, but the impression that phrase gives off to me is that Myungsoo's eyes widened in a stunning manner, which ends up sounding weird lol). Other than that, I really can't spot any other errors so you're great to go! You're also fine vocabulary-wise, but I realise you have the tendency to rely on adverbs sometimes. More to be explained in the Writing Style section.

Writing Style
Your writing style is as lovely to read as always; you insert humour when appropriate, and the ending lines are especially enlightening and heartwarming. Like I mentioned before, you tend to use adverbs too much, which makes the description rather vague since the whole emotion is reduced to just one word. Let me use an example. 
'She exclaimed proudly' - Yes, we do know Rika feels proud, but how proud is she? How does she show it? Maybe she stuck her chest up as she exclaimed in a loud, booming voice, as if shouting 'Look at that! It's my work! I painted such a masterpiece!'. Of course, you don't have to describe every single detail down to its core, because the redundancy may make the readers feel bored, but I feel that this is one part which you should emphasise on since it's clear that painting is really important in Rika's life. The more literal feel still lingers in your narration, so just take note of that. The recurring symbolism of a blank canvas is a great addition to this story, plus it's very fitting considering the fact that our two main characters love art. The only problem I have is the significance of this canvas in the story. More to be said in the Plot section!
P.S. I love your descriptions about the illustrations. Your words paint beautiful and vivid pictures. 

Characters
I think one major strength you have really lies in your characters. You just have the magical ability of making your characters seem so lovable just by their odd sense of humour (explains why everybody is in love with WooMi ; u ; I haven't read an actual story with them yet but I'll read Cheese Me soon! Anyways... Rika is such an optimistic girl, and it's a delight to read about her views towards art, and through them, to life itself. She is also absolutely (un)funny with Myungsoo, so we can't help but laugh at their antics. Although Myungsoo hasn't spoken much, the little moment when he did speak just brings fluffy feelings into my heart ; u ; Their chemistry is great; they can not only laugh at each other, but also exchange some of their deep thoughts pertaining to art.

Now, onto the not-so-good parts. I think it may have been due to the length of the story, but both characters are not sufficiently developed enough. Although Rika is absolutely lovely, she feels like a one-dimensional cheerful character who exists to cheer the ice prince up. There is no evident flaws she has (other than her lameness lol) other than the fact that she initially seemed skeptical about Myungsoo's win. I was actually hoping she might pull a jealousy card and ended up showing resentment towards Myungsoo, though she eventually understood his principles in the end (sounds cliche but it can work if you execute it well, and I believe in your skills haha), but the flow was all a bit too tight so that progression wasn't really fleshed out well. We also don't know how her ideologies were shaped, why happiness meant so much to her, or how art played a significant role in her life. It's like we're pulled into Rika's thoughts and starting to like her, before we're pulled out without getting to really know and love her. Myungsoo also doesn't have any other evident weaknesses other than the fact that he's really gloomy. There is growth in him, though the growth doesn't appear apparent nor significant. If he had been living his life with such a pessimistic mindset, how could one Boo Rika change things? How could she teach him to see the beauty of life (which is very broad and complicated)? In fact, their interactions seem rather limited (again, maybe due to the story length), so it's hard to be convinced that Rika made such a big difference. And I feel that as light-hearted as MyungRi's relationship is, there is very little depth. Maybe more interactions outside of the art room are needed lol.

I guess these flaws mostly arise because of the story's length, but flaws are still flaws regardless so hopefully you do consider these points in future! It depends on how you want to portray this story as. If writing a light-hearted story is your goal, then you've achieved that and you can probably disregard what I had mentioned (being too detailed can impede the 'light-heartedness' in your story, in fact). One problem that will occur, however, is that your story won't be as memorable. Also, their 'deep' moments appear deep but it feels like they're not going into anything extra serious (like they can talk about their personal experiences etc but they're just being very general about life - it doesn't give the readers any anchor events to hold to). 

Plot
The plot is fairly straightforward, albeit cliche. Your execution does wonders, but the unrealism factor still remains (how did Rika make the ever so pessimistic Kim Myungsoo break a smile? how can he change his mindset so quickly? change isn't very easy, sadly). There isn't anything new in terms of the storyline, yet the interactions bring life to this concept. Perhaps more solid experiences outside the art room would make the story a much better piece.

The meaning behind your story is profound; however, the lack of development of actual scenarios in the story makes it appear to be more of a 'message of the day' kind of thing rather than actual values we learn from the story. It's very inspirational, yes, but we also forget it easily. I think what I would like to see particularly is how Myungsoo's views change, how he manages to see the beauty in every little thing or how drastic perspectives can change something. Also, a deeper reflection about Myungsoo's contrasting views against Rika's would be nice. Although the story ends off with saying that it's up to us to decide how we should fill our blank canvas, the message seems to be ultimately positive - that we should fill our blank canvas with colours. But what if reality hits smack at you and is intent on erasing away all the colour left on your canvas? Would grappling for one last strand of hope be the best decision? Would smiling make you feel better, even when you're already so tired to the point when you don't feel like smiling anymore? I think that's how some people actually feel (one of my friends are like that too ;;). They see no point in feigning happiness, not when so much nonsense happens in life. That's just another side to a coin, and I feel your story has neglected to discuss about that aspect/provide a solid rebuttal to that aspect (lol this is giving me argumentative essay flashbacks). So your message just doesn't come off as strong as expected. Putting aside the character flaws, this may be one of the major flaws in this story, especially since you intend for it to be inspirational. It's still an uplifting read, but the intensity just isn't there.

Flow
It's a one-shot, so it's kinda expected, but I feel like we jump across the relationship development too fast. It's like they're just 'bam! I like you a lot' kind of thing. The progression is smooth so it's not too distracting, but it's just kinda weird considering we haven't had much development of their friendship/relationship beforehand. I can understand though ; u ;

Overall Enjoyment (7ish/10)
Although it seems like I gave a lot of criticism, I genuinely enjoyed this work! These flaws are actually not as noticeable as I make them seem to be. When I first read the story, I felt like I was actually insfired. But reading the story again and contemplating about the message of the story itself have caused me to wonder about some of the loopholes your story has left behind. This story as it is is a pleasant read, though it is lacking in some ways. I upvoted it anyways because I do like reading it, so that means something :)