begin{axis}[
    mesh/interior colormap name=hot,
    colormap={slategraywhite}{rgb255=(112,128,144) rgb255=(255,159,101)},
 ]

pgfplotsset{%
    colormap={slategraywhite}{rgb255=(112,128,144) rgb255=(255,159,101)}%
}%

documentclass[border=2pt]{standalone}

usepackage{pgfplots}
usepgfplotslibrary{colormaps}%

begin{document}
begin{tikzpicture}
begin{axis}[
    hide axis,
    xlabel=$x$,ylabel=$y$,
    mesh/interior colormap name=hot,
    colormap={slategraywhite}{rgb255=(112,128,144) rgb255=(255,159,101)},% <--- This works fine.
 ]
  addplot3 [domain=-1.5:1.5,surf, shader=faceted] {-exp(-x^2-y^2)};
end{axis}
end{tikzpicture}%
end{document}

usepackage{pgfplots}
usepgfplotslibrary{colormaps}%

pgfplotsset{%
    colormap={slategraywhite}{rgb255=(112,128,144) rgb255=(255,159,101)}%
}%
pgfplotsset{colormap/slategraywhite}% <-- activate colormap
pgfplotsset{colormap/blackwhite}

begin{document}
begin{tikzpicture}
begin{axis}[
    hide axis,
    xlabel=$x$,ylabel=$y$,
    mesh/interior colormap name=hot,
    colormap/slategraywhite,%         <--- How do I get this working.
 ]
  addplot3 [domain=-1.5:1.5,surf, shader=faceted] {-exp(-x^2-y^2)};
end{axis}
end{tikzpicture}%
end{document}