So many of the characters on the show are vastly different from those we remember from our childhood story books, for example Red Riding Hood is the wolf and Rumplestiltskin is both Belle's "Beast" and Captain Hook's (metaphorical) crocodile. For this reason it would have been possible to blind cast a lot of these characters instead of matching them to their mostly white Disney counterparts. However, a brief glance at the cast of Once Upon A Time looks a lot more like Disney than we even realized.
The following characters are the only minorities/people of color on the show (and a lot of them are already dead)...
Mulan, based on the Chinese legend "Hua Mulan" (which was the basis for the 1998 Disney film Mulan) is portrayed by Jamie Chung, who is Korean-American. Status: Alive.
Giancarlo Esposito, who is half black and half Italian, appears on the show as Sidney Glass (in Storybrooke) and the Magic Mirror (in the Enchanted Forest). Before he was cursed to become the Magic Mirror, he was the Genie of Agrabah. Status: Assumed to be living, but locked in Regina's basement.
Sir Lancelot (based on the character from the legend of King Arthur) is portrayed by Sinqua Walls, who is African-American. What's kind of cool about this character is that, supposedly, the casting call described him as African American. This means that they didn't just blind-cast, they were specifically looking for a person of color to play a character that is traditionally portrayed as white.
Unfortunately what's not cool about this character is that he has (so far) only appeared in one episode. At the end of the episode, we learn that Lancelot was killed long ago and he was really Cora using magic to appear as Lancelot. (So they finally introduce a black character and not only does he die in his first episode, but for half of the episode he's actually a white woman!) Status: Dead.
The Fairy Godmother, based on the character from "Cinderella" appears in one episode of the show and is played by Catherine Lough Haggquist. She is only present for a short amount of time before Rumplestiltskin kills her and takes her wand. Status: Dead.
Billy (the Storybrooke counterpart of Gus the mouse from "Cinderella") is played by Jarod Joseph and only appears in one episode in which he is killed by Cora. Status: Dead.
The dwarf Bashful appears in a handful of episodes, but is a very minor character. (The only dwarf who really gets a lot of screentime or dialogue is Grumpy.) He is portrayed by Mig Macario who is of Filipino descent. Status: Living.
Tamara, portrayed by Sonequa Martin-Green, is Neal's fiance who was on a secret mission to destroy magic. Or at least she thought, until she was killed in Neverland. Status: Dead.
"The Dragon" (portrayed by Chinese-American Tzi Ma) is a magical healer in Hong Kong that appears in one episode before being killed by Tamara. Status: Dead.
The actress that plays Regina Mills - Lana Parrilla - is Latina (of Sicilian and Puerto Rican descent) but her character is not Latina and the actress presents as white. Regina is arguably one of the most powerful characters on the show, both in Storybrooke and in the Enchanted Forest, but her ethnicity is never mentioned or even alluded to. Status: Alive, but hated by all.
Regina's father, the Valet (Prince Henry), was played by Tony Perez, but again, he presents as white and there is no indication as to whether the character is meant to be Latino/Hispanic. Status: Dead.
*Note: According to IMDB.com, there is also a "Lost boy" played by Alex Barima, who appears to be black but as of yet hasn't had any lines or much screentime. We won't hold our breath.*
Compare those few characters to the myriad of white characters and it's evident that Once Upon A Time may have a race problem. It's not clear whether this was an intentional choice or just TV's typical lack of diversity caused by a lack of diversity in writers/producers (e.g., creators Adam Horowitz and Edward Kitsis are both white). But whatever the reason, it's really unfortunate that the writers were given the opportunity to completely re-envision the fairy tales of our childhoods, yet didn't go so far as to add some diversity to the characters.