\pdfoutput=1

\setbox0=\hbox{\TeX}

\directlua0 {
  callback.register('define_font',
    function (name,size)
      if name == 'cmr10-red' then
        f = font.read_tfm('cmr10',size)
        f.name = 'cmr10-red'
        f.type = 'virtual'
        f.fonts = {{ name = 'cmr10', size = size }}
        for i,v in pairs(f.characters) do
          if (string.char(i)):find('[tacohanshartmut]') then
             v.width = tex.wd[0]
             v.commands = {
               {'special','pdf: 0 1 0 rg'},
               {'node',tex.box[0] },
               {'special','pdf: 0 0 1 rg'},
               {'down', 100000},
               {'node',tex.box[0] },
               {'special','pdf: 0 g'},
              }
          else
             v.commands = {{'char',i}}
          end
        end
      else
        f = font.read_tfm(name,size)
      end
      return f
    end
  )
}

\font\myfont = cmr10-red at 12pt \myfont  This is a line of text \par

\blank

\font\myfontx= cmr10 at 12pt \myfontx Here is another line of text \par

\bye