Update example of Lua filter for color span

[cderv]

For the next guy who comes here via google, here is a version of the lua script adapted to support hex-style RGB colors, and yes it also works with vanilla Pandoc:

--- copied from https://github.com/rstudio/rmarkdown-cookbook/issues/194#issuecomment-610614496
--- adapted to support hex-style RGB colors by pbsds
Span = function(span)
  color = span.attributes['color']
  -- if no color attribute, return unchange
  if color == nil then return span end

  -- tranform to <span style="color: red;"></span>
  if FORMAT:match 'html' then
    -- remove color attributes
    span.attributes['color'] = nil
    -- use style attribute instead
    span.attributes['style'] = 'color: ' .. color .. ';'
    -- return full span element
    return span
  elseif FORMAT:match 'latex' then
    -- remove color attributes
    span.attributes['color'] = nil
    -- encapsulate in latex code
    if string.sub(color, 1, 1) == "#" and #color == 7 then
      -- TODO: requires xcolor
      local R = tostring(tonumber(string.sub(color, 2, 3), 16))
      local G = tostring(tonumber(string.sub(color, 4, 5), 16))
      local B = tostring(tonumber(string.sub(color, 6, 7), 16))
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor[RGB]{'..R..','..G..','..B..'}{')
      )
    elseif string.sub(color, 1, 1) == "#" and #color == 4 then
      -- TODO: requires xcolor
      local R = tostring(tonumber(string.sub(color, 2, 2), 16) * 0x11)
      local G = tostring(tonumber(string.sub(color, 3, 3), 16) * 0x11)
      local B = tostring(tonumber(string.sub(color, 4, 4), 16) * 0x11)
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor[RGB]{'..R..','..G..','..B..'}{')
      )
    else
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor{'..color..'}{')
      )
    end
    table.insert(
      span.content,
      pandoc.RawInline('latex', '}')
    )
    -- returns only span content
    return span.content
  else
    -- for other format return unchanged
    return span
  end
end

Originally posted by @pbsds in https://github.com/rstudio/rmarkdown-cookbook/issues/194#issuecomment-1111027768

[CLRafaelR]

In related to #323, I would like to suggest the following modification so that we can use @pbsds's implementation of hex-RGB specification for revealjs and beamer, too.

Span = function(span)
  color = span.attributes['color']
  -- if no color attribute, return unchange
  if color == nil then return span end

  -- tranform to <span style="color: red;"></span>
  if FORMAT:match 'html' or FORMAT:match 'revealjs' then -- Also available for revealjs
    -- remove color attributes
    span.attributes['color'] = nil
    -- use style attribute instead
    span.attributes['style'] = 'color: ' .. color .. ';'
    -- return full span element
    return span
  elseif FORMAT:match 'latex' or FORMAT:match 'beamer' then -- Also available for beamer
    -- remove color attributes
    span.attributes['color'] = nil
    -- encapsulate in latex code
    if string.sub(color, 1, 1) == "#" and #color == 7 then
      -- TODO: requires xcolor
      local R = tostring(tonumber(string.sub(color, 2, 3), 16))
      local G = tostring(tonumber(string.sub(color, 4, 5), 16))
      local B = tostring(tonumber(string.sub(color, 6, 7), 16))
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor[RGB]{'..R..','..G..','..B..'}{')
      )
    elseif string.sub(color, 1, 1) == "#" and #color == 4 then
      -- TODO: requires xcolor
      local R = tostring(tonumber(string.sub(color, 2, 2), 16) * 0x11)
      local G = tostring(tonumber(string.sub(color, 3, 3), 16) * 0x11)
      local B = tostring(tonumber(string.sub(color, 4, 4), 16) * 0x11)
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor[RGB]{'..R..','..G..','..B..'}{')
      )
    else
      table.insert(
        span.content, 1,
        pandoc.RawInline('latex', '\\textcolor{'..color..'}{')
      )
    end
    table.insert(
      span.content,
      pandoc.RawInline('latex', '}')
    )
    -- returns only span content
    return span.content
  else
    -- for other format return unchanged
    return span
  end
end