documentclass{standalone}
usepackage{pgfplots}

begin{document}

begin{tikzpicture}
begin{axis}[colorbar, colormap={}{ gray(0cm)=(1); gray(1cm)=(0);}]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

begin{tikzpicture}
begin{axis}[colorbar, colormap={}{ gray(0cm)=(0); gray(1cm)=(1);}]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

end{document}

documentclass{standalone}
usepackage{pgfplots}
usepgfplotslibrary{colormaps}

begin{document}

pgfplotsset{
    %colormap={X}{ gray(0cm)=(1); gray(1cm)=(0);},
    colormap/winter,
}
begin{tikzpicture}
begin{axis}[colorbar]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

begin{tikzpicture}
begin{axis}[colorbar,
    colormap={reverse winter}{
        indices of colormap={
            pgfplotscolormaplastindexof{winter},...,0 of winter}
    },
]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

end{document}

documentclass{standalone}
usepackage{pgfplots}
usepgfplotslibrary{colormaps}

makeatletter
defcustomrevertcolormap#1{%
    pgfplotsarraycopy{pgfpl@cm@#1}to{custom@COPY}%
    c@pgf@counta=0
    c@pgf@countb=pgfplotsarraysizeof{custom@COPY}relax
    c@pgf@countd=c@pgf@countb
    advancec@pgf@countd by-1 %
    pgfutil@loop
    ifnumc@pgf@counta<c@pgf@countb
        pgfplotsarrayselect{c@pgf@counta}of{custom@COPY}topgfplots@loc@TMPa
        pgfplotsarrayletentryc@pgf@countdof{pgfpl@cm@#1}=pgfplots@loc@TMPa
        advancec@pgf@counta by1 %
        advancec@pgf@countd by-1 %
    pgfutil@repeat
%pgfplots@colormap@showdebuginfofor{#1}%
}%

makeatother

begin{document}

pgfplotsset{
    %colormap={X}{ gray(0cm)=(1); gray(1cm)=(0);},
    colormap/winter,
}
begin{tikzpicture}
begin{axis}[colorbar]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

begin{tikzpicture}
%customrevertcolormap{X}
%customrevertcolormap{jet}
customrevertcolormap{winter}
begin{axis}[colorbar]
addplot[mesh,thick] {x^2};
end{axis}
end{tikzpicture}

end{document}