Plenty of times in films - such as Thor - the issue of race causes some people to get up in arms on one side of some supposed fence or another. This has seemed to especially plague The Hunger Games and now Catching Fire, more so than pretty much any other movie that comes to mind.
When Jennifer Lawrence was announced as Katniss, she drew criticism for being an older blonde white girl with curves instead of being a scrawny olive-skinned 16-year-old with black hair. Makeup and movie magic of course transformed her into a character closer to what is described in the books but the question still remained, why couldn't they have just cast someone who had those characteristics to begin with?
When Amandla Stenberg (and to a lesser degree, Dayo Okeniyi) were revealed as Rue and Thresh there was a well publicized backlash against them being black. The issue came up once again when the film was released as those who didn't follow the casting saw the characters for the first time. Despite being two of the most specifically described characters in the book as having very dark skin, there were complaints ranging from surprise to claiming that Rue not being "an innocent blonde white girl" ruined the movie.
The casting of Lenny Kravitz as the beloved Cinna was, I think, a surprise overall - he just wasn't someone most people had imagined in the part. Once again, the fact that he has black skin seemed to upset some people - not that he was a left field casting choice or that he had almost zero acting experience despite the fact that Gary Ross specifically sought him out because he fulfilled the vision he had for the character. Cinna's physical description is very ambiguous (as are many of the characters) in the book, with skin being a little darker than Katniss's being about as specific as it got. That could really mean he spent a lot of time in the tanning salon (he was a stylist after all) or was middle eastern, or mixed race, or really, just about anything but pale. It seemed though, once again, that many people assumed he was white and some were all kinds of distraught at the thought of Cinna being anything but.
Now, as we get into the casting of the sequel, Catching Fire, the racial fires are flaring once again. Particularly as fans discuss who should play Finnick. Darker skinned Jesse Williams has come out as a fan favorite (despite having never heard of the character before he heard about the fans who wanted him in the part) and immediately there are those who say he in no way fits the description of being "extremely handsome, tall, muscular, and athletic, with tan skin, bronze-colored hair, and incredible sea-green eyes." Sometimes it's the fact that he's not white that gets cited as the reason, other times he's just not handsome enough in the eyes of a particular fan. Once again, "tan skin," and "bronze-colored hair" are somewhat ambiguous terms - Suzanne Collins seems to like to leave much up the readers' imagination.
What is it about these characters that seems to bring out all sorts of racist comments from people? Here's what I think - Suzanne Collins has crafted characters that people care about. They care about them a lot. And as you read a story, you form a picture of them in your head and that picture becomes that character. Especially if they are described in more ambiguous terms, it's only logical that picture will more often than not be someone who looks like you - whether you are white, black, Native American, Chinese, Mexican, or bright purple with six-foot wings.
When that character comes to life in a movie, there is a good chance they won't perfectly fit the vision in your head. They may be taller, older, or a completely different race either because that's what the director saw in their head or because the actor or actress is just a perfect fit with their personality and talent, regardless of if they look quite like what people might expect or not.
I'll admit - I pictured Finnick, as well as most other characters as white. It's something that happens automatically, I think, because of my race and the fact that I grew up in an area that was not terribly diverse. As long as an actor captures the essence of the character, I couldn't care less what the color of their skin is. In Finnick's case, as long as his skin looks like someone who spent his life on the shore, fishing, it doesn't really matter that means he's got a surfer dude's golden tan, a native Hawaiian's darker skin, Jesse William's racial background, or something else.
It is important to remember that just like you have a set-in-stone picture of that character in your head, so does everyone else, and it probably doesn't match up with what yours looks like, but it's just as right. Unless you try to tell Suzanne Collins that Rue is not actually black, despite what she wrote. Then you are wrong.