This site suggests potassium chloride for purple flames:
https://www.madisonco.com/fabulous-fun-facts-how-turn-fire-different-colors.
Haven’t seen the movie, but the colour of ordinary flames can surely be altered with either practical effects like a colour filter or with CGI, or the flames might be entirely CGI.
We have a wood burning stove, and I put these in sometimes for a bit of variety:
They make the flames go green or blue depending on whether the pine cone is coated with copper II sulphate (green flames) or with copper chloride (blue flames). That’s pretty safe, and they last around 5 minutes.
[Edit: follow-up -
How to Make Purple Fire (sciencenotes.org) recommends a blend of potassium chloride and strontium nitrate from a red-flamed road flare, which is not something I've seen for sale in the UK. Last time I cut up fireworks, and separated and burned small quantities of the contents was in school, inside a fume cupboard, under the close supervision of a chemistry teacher. Generally it seemed to be quite safe, even with little piles of loose gunpowder and magnesium, but there's no way I'd mess about with flares at home, not even outside. Once they are lit, they're really hard to put out, particularly as you should not be holding them. If you want to do this, take a chemistry class and have the teacher approve what you propose to do, use all the appropriate safety equipment and be supervised if you aren't already qualified to do stuff like this. Losing your eyebrows might seem like a badge of honor, losing a thumb or an eye, your house or your life because you did something foolish, not so much.]
