Yes, I've found the error.
Thank you John for your help.
The pandoc-discuss group is very helpful.

On Wednesday, May 6, 2020 at 6:42:56 PM UTC+2, John MacFarlane wrote:
>
>
> Probably attributes.text-decoration needs to be 
> attributes["text-decoration"] -- the hyphen is 
> interpreted as subtraction. 
>
> "R. Wils" <rai...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org <javascript:>> writes: 
>
> > Yes!! Thank you very much. 
> > 
> > It worked also converting to PDF (adding -o mydoc.pdf to the call with 
> > pandoc) 
> > 
> > I tried to add text underline to the same lua function; 
> > 
> > function Span(el) 
> >   if FORMAT == "latex" and el.attributes.color then 
> >     return { pandoc.RawInline("latex", "\\textcolor{" .. 
> >   el.attributes.color .. "}"), el } 
> >   elseif FORMAT == "latex" and el.attributes.text-decoration then 
> >     return { pandoc.RawInline("latex", "\\underline{" .. 
> >   el.attributes.text-decoration .. "}"), el } 
> >   end 
> > end 
> > 
> > with input 
> > [this is my text]{text-decoration=underline} 
> > 
> > This gives an error `my_luafilter.lua:5: attempt to perform arithmetic 
> on a 
> > nil value (field 'text')` 
> > What did I wrong? 
> > 
> > 
> > On Tuesday, May 5, 2020 at 10:59:17 PM UTC+2, John MacFarlane wrote: 
> >> 
> >> 
> >> Try this one: 
> >> 
> >> function Span(el) 
> >>   if FORMAT == "latex" and el.attributes.color then 
> >>     return { pandoc.RawInline("latex", "\\textcolor{" .. 
> >>   el.attributes.color .. "}"), el } 
> >>   end 
> >> end 
> >> 
> >> Call with pandoc --lua-filter my_luafilter.lua -t latex 
> >> 
> >> With input 
> >> 
> >> [this is my text]{color=red} 
> >> 
> >> (note = not :) you should get the right result. 
> >> 
> >> 
> >> "R. Wils" <rai...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org <javascript:>> writes: 
> >> 
> >> > Thank you very much for your reply. 
> >> > 
> >> > Pandoc is so nice but also very difficult to understand. 
> >> > I've never seen the lua language before. I'm quit familiar with 
> Python 
> >> and 
> >> > Html but I've never seen lua code before. 
> >> > 
> >> > I've read the pages: 
> >> > - lua-filters.html 
> >> > Created a file: my_luafilter.lua 
> >> > 
> >> > with your function inside 
> >> > 
> >> > function Span(el) 
> >> >   if el.attributes.color then 
> >> >     return { pandoc.rawInline("latex", "\\textcolor{" .. 
> >> >   el.attributes.color .. "}"), el } 
> >> >   end 
> >> > end 
> >> > 
> >> > And in my pandoc.markdown file I added this: 
> >> > 
> >> > <span style="color:red">test</span> 
> >> > 
> >> > And added this filter to my pandoc command: 
> >> > --lua-filter=d:\\my_luafilter.lua 
> >> > 
> >> > Nothing happened. 
> >> > What did I wrong? 
> >> > 
> >> > Sorry to ask you all these questions 
> >> > I just need *colored text* and *underscore* for a markdown document I 
> >> > created and which I want to convert to PDF format. 
> >> > I'll learn every day a bit more about lua and lua filters. 
> >> > 
> >> > On Tuesday, May 5, 2020 at 6:15:06 PM UTC+2, John MacFarlane wrote: 
> >> >> 
> >> >> 
> >> >> Cleanest way is to use a Span with an attribute, plus a 
> >> >> lua filter that reacts to Spans with that attribute by 
> >> >> adding some LaTeX color commands. 
> >> >> 
> >> >> See the lua filter documentation and feel free to ask 
> >> >> if you have questions. 
> >> >> 
> >> >> Something like this (untested) 
> >> >> 
> >> >> function Span(el) 
> >> >>   if el.attributes.color then 
> >> >>     return { pandoc.rawInline("latex", "\\textcolor{" .. 
> >> >>   el.attributes.color .. "}"), el } 
> >> >>   end 
> >> >> end 
> >> >> 
> >> >> "R. Wils" <rai...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org <javascript:>> writes: 
> >> >> 
> >> >> > Does anyone know how to color text in a PDF 
> >> >> > (converting from a pandoc.markdown text file)? 
> >> >> > 
> >> >> > I tried to use a span and div tag without success. 
> >> >> > 
> >> >> > (Sorry I'm a new pandoc user) 
> >> >> > 
> >> >> > -- 
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> >> >> > To view this discussion on the web visit 
> >> >> 
> >> 
> https://groups.google.com/d/msgid/pandoc-discuss/5f7aae6a-8087-439d-874b-dcf547dd10b1%40googlegroups.com. 
>
> >> 
> >> >> 
> >> >> 
> >> > 
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> >> 
> >> 
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