How to define specific styles for each tick?

Question

I want to change the style (such as color, font, size and shape of tick mark) for each tick.

In this example I want to change the color of the numbers (2,3,4) to something different and define a specific tick mark for each of them.

\documentclass[10pt]{article}
\usepackage{pgf,tikz,pgfplots}
\pgfplotsset{compat=1.5}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\pagestyle{empty}

\begin{document}

 \end{tikzpicture}


\begin{tikzpicture}
    \begin{axis}[
        x=1.5cm,y=1cm,
        axis lines=middle,
        ymajorgrids=true,
        xmajorgrids=true,
        xmin=-4,
        xmax=4,
        ymin=-4,
        ymax=4,
        xtick={-9,-8,...,9},
        ytick={-5,-4,...,6},]
        \clip(-5,-5) rectangle (5,5);
    \end{axis}
\end{tikzpicture}

\end{document}

Answer 1

\documentclass[10pt]{article}
\usepackage{pgf,tikz,pgfplots}
\pgfplotsset{compat=1.5}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\pagestyle{empty}

\begin{document}
    



\begin{tikzpicture}
    \begin{axis}[
        x=1.5cm,y=1cm,
        axis lines=middle,
        ymajorgrids=true,
        xmajorgrids=true,
        xmin=-4,
        xmax=4,
        ymin=-4,
        ymax=4,
        xtick={-9,-8,...,9},
        ytick={-5,-4,...,6},
        extra x ticks={4,3,2,1},
        extra x tick labels={4,3,2,1},
        extra x tick style={
            xticklabel style={yshift=0.5ex, anchor=south, color=red, font=X}}]
        \clip(-5,-5) rectangle (5,5);
    \end{axis}
\end{tikzpicture}

\end{document}

a second example

\documentclass[10pt]{article}
\usepackage{pgf,tikz,pgfplots}
\pgfplotsset{compat=1.5}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\pagestyle{empty}

\begin{document}
    



\begin{tikzpicture}
    \begin{axis}[
        x=1.5cm,y=1cm,
        axis lines=middle,
        ymajorgrids=true,
        xmajorgrids=true,
        xmin=-4,
        xmax=4,
        ymin=-4,
        ymax=4,
        xtick={-9,-8,...,9},
        ytick={-5,-4,...,6},
        extra x ticks={4,2},
        extra x tick labels={$2\pi$, $4\pi$},
        extra x tick style={
            xticklabel style={yshift=0.5ex, anchor=south}}]
        \clip(-5,-5) rectangle (5,5);
    \end{axis}
\end{tikzpicture}

\end{document}