Is it just a coincidence that that's exactly what I would have guessed by looking at the color of feathers in the fossil? Am I falling prey to hindsight bias?
Hmm.. TBH, I don't think anyone can tell you that! However, after taking a look at it, I myself may have suggested reddish feathers along the neck and back, with some kind of red/purple coloration going down the tail (though I have no significant expertise in this field).
Or is it the case that the feathers are so well preserved that their color is visible to the naked eye?
Melanosomes are actually fairly large; about 500 nm (source). Relatively speaking, this is somewhere between the size of the flu virus, and of mitochondria; see here for scale. According to this book, written by the Swiss Institute of Bioinformatics, it states:
... besides [zebrafish] being vertebrates like us - their epidermis and their eggs are transparent, and embryonic development can be overved and tinkered with at every stage. Likewise, even melanocytes and their melanosomes are visible to the naked eye.
So, it would seem that it's possible to observe melanosomes with the naked eye, given a high enough (melanosome) concentration. It should also be noted though that, although the fossil did preserve the melanosomes (here), I'm not necessarily sure that that means the color observed in the fossil is the same color that would have been observed in the living organism. If anyone can comment/add to this, that would be quite welcomed.
Here's an image that suggests what this dinosaur would look like, given the discovery and analysis of the Sinosauroterpyx's melanosomes, acquired from the fossil you provided a picture of.