Colouring cells in a table

mcode is not in the standard distributions but you can see the same effect using xcolor

documentclass[a4paper,12pt]{extarticle}
%% Language and font encodings
usepackage[english]{babel}
usepackage[utf8x]{inputenc}
usepackage[T1]{fontenc}

%% Sets page size and margins
usepackage[a4paper,top=2.5cm,bottom=2.5cm,left=2.5cm,right=2.5cm,marginparwidth=1.75cm]{geometry}

usepackage{amsmath}
usepackage{amsfonts} 
usepackage{amssymb}
usepackage{amsthm}
usepackage{mathtools}
usepackage{optidef} 
usepackage{xcolor}
usepackage[table]{xcolor}

begin{document}
    begin{tabular}{ | l | l | l | }
      hline
      A & B & C 
      hline
      D & E & cellcolor{green}F 
      hline
      G & H & I 
      hline
    end{tabular}
end{document}

this produces the error

! LaTeX Error: Option clash for package xcolor.

and if you scroll past that the error

! Undefined control sequence.
l.24       D & E & cellcolor
                             {green}F 
? 

as cellcolor is not defined you get the output that you show.

Note that after any error the PDF output is not intended to be usable, it is at best, usable as a debugging aid. Never ignore error messages.

To fix the option clash, load xcolor earlier with the option, so simply swap the lines:

documentclass[a4paper,12pt]{extarticle}
%% Language and font encodings
usepackage[english]{babel}
%usepackage[utf8x]{inputenc}
usepackage[T1]{fontenc}

%% Sets page size and margins
usepackage[a4paper,top=2.5cm,bottom=2.5cm,left=2.5cm,right=2.5cm,marginparwidth=1.75cm]{geometry}

usepackage{amsmath}
usepackage{amsfonts} 
usepackage{amssymb}
usepackage{amsthm}
usepackage{mathtools}
usepackage{optidef} 
usepackage[table]{xcolor}
usepackage{xcolor}

begin{document}
    begin{tabular}{ | l | l | l | }
      hline
      A & B & C 
      hline
      D & E & cellcolor{green}F 
      hline
      G & H & I 
      hline
    end{tabular}
end{document}