\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
beginfig(1);
  path ray; ray = origin -- 200 right;
  numeric theta; theta = 42;
  draw ray;
  draw ray rotated 1/3 theta withcolor 2/3 red;
  draw ray rotated 2/3 theta withcolor 2/3 red;
  draw ray rotated theta;
  dotlabel.llft("$0$", origin);
  label("$\theta/3$", 72 right rotated 1/6 theta) withcolor 2/3 blue;
  label("$\theta/3$", 72 right rotated 3/6 theta) withcolor 2/3 blue;
  label("$\theta/3$", 72 right rotated 5/6 theta) withcolor 2/3 blue;
endfig;
\end{mplibcode}
\end{document}