I found this on Reddit:
A lot of people seem to think the colors are about difficulty. They're actually about how straightforward they deem the category, and there's a pretty clear pattern. (This is all based on memory, so there are probably common themes I'm leaving out, but this is the gist.) Yellow is almost always synonyms. Even if the words are obscure, synonyms will still be yellow. A second synonym category will be green, but green is often members of a group. If a category is components of something, even a very common object, that will usually be blue. And purple usually requires e…