I got it. The filter you're using performs a search on the "style" attribute's value even if there is none. The error happens when citeproc is called because citeproc creates spans with no "style" attribute.
Simply replace l. 2 with the following:
if el.attributes.style and string.find(el.attributes.style, "color") then
Le Saturday 10 July 2021 à 10:31:53AM, dualer a écrit :
> Sorry, I forgot something. I have added `-C --bibliography=Library.bib`
> to the cmd code.
>
> 在2021年7月11日星期日 UTC+8 上午12:36:49<dualer> 写道：
>
> Hi everyone! I have a naive question: When I transform .md to .pdf,
> how to keep the font color unchange? I have
> see (https://stackoverflow.com/questions/62831191/using-span-for-font-color-in-pandoc-markdown-for-both-html-and-pdf),
> but the code don't work when meeting following markdown code:
>
> <span style="color: red;">@red text</span>
>
> The key point above is `@`, and it interrupts `lua filter` to work,
> but I need it when I add some references, e.g. [@XX]. So how to solve
> it?
>
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