Inspired by this question, are questions that ask about a specific detail (like, for example, the color of a creature's blood) or specific incident (like how a specific action could occur) on topic when the question springs from a novel (or other work of fiction) that takes place in a role-playing game setting? Or should such questions be migrated to a different stack?
One thing to note in this particular case: WH40k is predominantly not an RPG setting, it's a model wargaming setting. The setting itself has since taken on a life of its own, where it has become an piece of IP independent of any particular expression thereof. The novels and (multiple) RPGs aren't even made by the same companies.
Consequently, the novels aren't even novels about an RPG setting, they're novels set in an independent setting, which happens to have RPGs set in it.
This is equivalent to getting questions about Star Wars novels here.
Only if the question is about RPGs or related to RPGs
The question that inspired this is about a setting that happens to have RPGs set in it, but is not an RPG setting. The novels have no particular relationship to RPGs, and nor does the question. The question is asking about a setting detail in a science-fiction novel.
As such, it is firmly off topic, just like a question about a character's words in Vector Prime would be off-topic. Fortunately, it's in the domain of Science Fiction and Fantasy Stack Exchange, so unlike many off-topic questions we get, it has a home in the network to go to.
If the question had been seeking information for using it in an RPG, that would have been on-topic (I hope nobody is going to go and make that question-laundering edit to the question now, though). Our minimum bar for “is it about RPGs?” isn't hard to clear. But it's merely a question about a plot detail in a novel unrelated to RPGs.
A development: off-topic confirmed
The asker has clarified that they're asking about armour painting and blood colour because they're thinking about how to paint their WH40k wargaming miniatures, for wargaming. They've been directed to Board & Card Games SE, where they're at risk of being shut down again because lore questions are marginal there (emphasis mine):
[…] if someone started asking questions about plot points that are best accessed through books, I might advise them to use the SciFi/Fantasy Stack Exchange instead. I think an analogous situation would be if someone asked questions about, say, making a custom table for board gaming. I think a lot of questions about its functionality and design would be on-topic here, but if they started asking what kind of glue to use for the joints there's probably a construction or DIY stack exchange better suited.
(Though being marginal, maybe it'll be fine there.)
This development confirms that the question about the character's words in the novel is solidly off topic here. Their question would be helped best by the setting-lore experts that populate Sci-Fi SE.
(I would do a migration right now, except that doing so in the middle of a discussion would be presumptuous. However, I'm concerned that in practical terms this discussion is now hindering the user getting expert help as soon as possible.)
If it fits into the category of [lore] question, then maybe.
We have both [lore] and [history-of-gaming] question categories, and tags, that if asked well are often acceptable questions. That said, there are a variety of [history-of-gaming] questions, and [lore] questions, that get closed due to not being well asked questions. I thus recommend that examining the question to see if it is well asked, like any other, be the first criteria used before seeing how well it fits into the [lore] or [history-of-gaming] class.
Is there an element of what's going on at the table in the question?
A further examination point would be to see if the question is being asked in aid of folding such literary/lore details into game play: while that's a more on topic question, there are some [lore] questions that do not necessarily get asked in a "help me with this at table issue" that are good questions.