documentclass[tikz,border={12pt,12pt}]{standalone}
usepackage{pgfplots}
usetikzlibrary{plotmarks}
pgfplotsset{compat=newest}
begin{document}
begin{tikzpicture}

 begin{axis}[hide axis,colorbar,colormap/jet,
  colorbar style={yshift=-2cm,
    xtick={18,22,26,30,34,38,42,45},
    xticklabel={18,22,26,30,34,38,42,45},
    rotate=270,
    yticklabel pos=left,
    yticklabel style={anchor=south},},
    ]
   {
  };
 end{axis}
end{tikzpicture}

end{document}

documentclass{standalone}
usepackage{pgfplots}
begin{document}
begin{tikzpicture}
begin{axis}[
    hide axis,
    scale only axis,
    height=0pt,
    width=0pt,
    colormap/jet,
    colorbar horizontal,
    point meta min=18,
    point meta max=45,
    colorbar style={
        width=10cm,
        xtick={18,20,25,...,45}
    }]
    addplot [draw=none] coordinates {(0,0)};
end{axis}
end{tikzpicture}

end{document}

documentclass{standalone}
usepackage{pgfplots}
begin{document}
begin{tikzpicture}
pgfplotscolorbardrawstandalone[
    colormap/jet,
    colorbar horizontal,
    point meta min=18,
    point meta max=45,
    colorbar style={
        width=10cm,
        xtick={18,20,25,...,45}}]
end{tikzpicture}

end{document}