outline.tex

\documentclass{article}
\usepackage{fontspec,xcolor,luamplib}
\setmainfont{tex gyre pagella}
\directlua{
  local function ol(f,_,v)
    v = tonumber(v) or 3
    v = v * f.size / 6578.176 % 3 means 0.3pt when using 10pt font
    f.width = v
    f.mode = 1
  end
  fonts.constructors.features.otf.register {
    name = 'outline',
    description = 'outline glyphs',
    manipulators = {
      base = ol,
      node = ol,
      plug = ol,
    }
  }
}
\newfontfeature{outline}{outline\ifx\empty#1\empty\else=#1\fi}
\ExplSyntaxOn
\NewDocumentCommand\drawoutline{ O{3} m +m }
{
  \color_group_begin:
  \addfontfeature{outline=#1}
  \color_stroke:n { #2 }
  #3
  \color_group_end:
}
\NewDocumentCommand\filldrawoutline{ O{3} m m +m }
{
  \color_group_begin:
  \addfontfeature{FakeBold=#1}
  \color_fill:n   { #2 }
  \color_stroke:n { #3 }
  #4
  \color_group_end:
}
\ExplSyntaxOff
\begin{document}

\drawoutline{red!50!yellow}
{\Huge Lorem ipsum dolor sit amet,}

\bigskip

\filldrawoutline[2.5]{red!50!yellow}{blue}
{\Huge Lorem ipsum dolor sit amet,}

\begin{mplibcode}
  beginfig(1)
  draw btex
    \addfontfeature{FakeBold=3}
    Lorem ipsum dolor sit amet
  etex
    scaled 2
    rotated 15
    withcolor 1
    withprescript "MPlibOverrideColor=1 .5 0 rg 0 0 1 RG";
  endfig;
\end{mplibcode}

\end{document}