Changing text color for charts using pgf-pie

I can’t really recommend pgf-pie due its inflexibility (and its use of the main PGFkeys path) but here is a etoolbox powered solution to insert a key that is tried on the node that includes the number (and only that number).

\documentclass[tikz]{standalone}
\usepackage{pgf-pie,etoolbox}
\makeatletter
\patchcmd\pgfpie@slice
  {node {\scalefont{#3}\beforenumber#3\afternumber}}
  {node[/every only number node/.try] {\scalefont{#3}\beforenumber#3\afternumber}}{}{}
\makeatother
\begin{document}
\begin{tikzpicture}[scale=0.9]
\pie[
  /tikz/every pin/.style={align=center},
  every only number node/.style={text=white},
  text=pin,
  rotate=240,
  explode=0.2,
  color={blue!70,cyan!70,red!70,orange!50}
] {65/Amplificatori di\\potenza, 10/Elaborazione\\del segnale,17.5/Sistemi di\\raffreddamento, 7.5/Alimentazione}
\end{tikzpicture}
\end{document}

Output