I get initially annoyed when any adaptation on tv doesn’t match my mental picture of it from books, but I’ve learned to get over myself most of the time. Usually the biggest differences are around plots and personality which is the most annoying thing. Sometimes I get annoyed when a character’s appearance is so different from the book, when it’s something that’s very heavily emphasised in the book. I end up needing to see the tv version as an alternative universe version.
So by my personal rules, which I’m sure feel different for everyone:
Little mermaid - race totally irrelevant as long as she’s a mermaid. Her mermaidiness is her key feature.
Snow White - one of the key recurring lines of the very old story is black hair, white skin and red lips. So anyone of any race or background who looks like that would be fine, if you’re going to call it “Snow White”. However, there are other very old variants of this tale from around the world and across many different cultures, eg there’s an Indonesian poem version from 1750 where the character Syair Bidasari is described here “her cheeks are described as like ‘the bill of a flying bird’, her nose ‘like a jasmine bud’, her pretty face like ‘the yellow of an egg’, and her teeth like ‘a bright pomegranate.’ “
https://www.pookpress.co.uk/project/snow-white-history/
So a more interesting updating would be using one of these alternative stories, rather than clumsily switching out.
Goldilocks should have long golden hair, but rapunzel could have any appearance as long as the hair is stupidly long. Etc etc.
I do think it’s worth introducing some anachronisms into historical dramas purely for the sake of giving non-white actors a broader and larger range of roles. Particularly because we have so much historical drama tv in the UK - it’s a huge market. But it’s best to make sure that there’s internal consistency - so if there was a big plot point around lineage and background, and one of the characters looked completely out of type, that would be confusing. I’m a bit worried about the new Bernard Cornwell King Arthur adaptation, for instance, where Merlin is played by a black actor. In the Bernard Cornwell book series, Merlin is fanatically obsessed with expunging all non native Briton people from the British isles (Saxons, romans etc) and restoring the old British gods. It’s uncomfortably like a plan for ethnic cleansing, frankly! So him being of a totally different ethnic background is very jarring. In any other King Arthur adaptation, where Merlin’s role is different and his motivations are different, it wouldn’t necessarily be nearly so odd. On the other hand, the book already has a black Knight / warlord, so introducing more characters of different races in that way would work really well.
And saffron - has definitely been grown in England for many centuries for use in cooking and dyeing. Croydon is likely named after the saffron fields. Several medieval recipes here from the British museum: https://www.britishmuseum.org/blog/how-cook-medieval-feast-11-recipes-middle-ages