\begin{tikzpicture}
  \pie[color={black!10, black!20, black!30, black!40}]
  {10/A, 20/B, 30/C, 40/D}

  \pie[pos={8,0}, color=blue!20] {10/A, 20/B, 30/C, 40/D}
\end{tikzpicture}