Thanks Albert,

Yes it works.

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

But something is strange:

Input:

[this is my text]{color=red}  
[this is my text]{text-decoration=underline}

Output:

this is my text
underlinethis is my text

It underlines the word underline!

Is my input wrong?