\pdfcompresslevel=0
\documentclass[a4paper]{article}

\usepackage{pgfplots}


\begin{document}

\pgfplotsset{
	colormap default colorspace=gray,
	% re-create default colormap:
	colormap/hot,
	samples=4,
	colorbar,
}
\begin{tikzpicture}
	\begin{axis}
	\addplot3[surf] {x*y};	
	\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
	\begin{axis}[colorbar sampled={surf,shader=interp}]
	\addplot3[surf,shader=interp] {x*y};	
	\end{axis}
\end{tikzpicture}
\end{document}
