This is actually a generational shift in the readership. (I say as a fan who started reading the very first trilogy when I was in middle school, when only the first 3 books were out. I'm in my 30s now.) It's been pretty cool to witness.
When the first books were published, there wasn't as much talk about diversity in the SF&F fandom, and it wasn't phrased or even approached/talked about in the same way. It wasn't as nuanced. We've made great strides on the topic! Also, the internet was in its infancy, so people who were otherwise open-minded hadn't been on the internet or social media as much and weren't exposed to what discussions on diversity that WERE around. Like, if you didn't think to go looking for it (and your average fan just wanting to get fanart might not), you wouldn't necessarily come across it. So, a lot of the recent discussions and recent criticism and analysis about this stuff hadn't even taken place then, or wasn't as widespread or accessible, and because Hobb was somewhat vague and left some of it open to interpretation, fans made fanart based on what was in their head, and the topic of diversity was not really in the heads of a lot of fans at the time.
So, fanart made earlier in the series' run will often interpret the "dark" Buck people as being somewhat Spanish or Greek-looking, vs. the blonder Mountain people, or the pretty-much-albino Fool. Still "white" though. So Buck people were usually shown in official art and in fanart as dark-haired, dark-eyed, with MAYBE something of a tan, but not usually brown-skinned. This is because the "white as default" assumption was not as questioned by the SF&F fandom as a whole, and the interpretation of descriptions like "dark" were taken much less literally. If skin color wasn't very strictly spelled out, readers would often default to "white" in their head, even if they weren't white themselves, because pretty much all the characters in media were always white and you build a head-template off of that that you unconsciously apply.
However, since this IS a long-running series, and the average person's exposure to this stuff has changed as society has taken a much harder look at it, you'll see many younger fans and artists take the description of "dark" much more literally these days, and purposely break the "white as default" assumption. This is because fandom is much more educated on this topic than it used to be. It's pretty much impossible to be a part of fandom and not encounter nuanced, detailed, analytical discussions about diversity and race. So newer fanart is often much more diverse, and is much more likely to show Fitz as a guy with actually brown skin, and interpret what would have been seen as "loose" curls earlier in the series' run as tight afro-style curls, because the way Robin Hobb wrote was vague enough that you could interpret it as either way.
So check the publication/creation date of fanart. The older stuff is much more likely to show white Fitz, while the more recent has seen an uptick in dark-skinned Fitz.
Like I said, it's been cool to watch this develop, and it's been cool to see what the 2nd generation to encounter Hobb's work thinks of it. In some ways, fandom is very similar to what it was in 2000, 2001, and in other ways rather different.