Are beamer, hhline, and [table]{xcolor} incompatible?

as samcarter commented

\documentclass[xcolor=table]{beamer}
\usepackage{makecell, multirow}
    \renewcommand\theadfont{\normalsize\bfseries\color{white}}
    \renewcommand{\theadgape}{}
\usepackage{hhline}
\makeatletter
\let\zz\reset@color
\def\reset@color{\kern\z@\zz}
\makeatother

\begin{document}
\begin{frame}
\frametitle{test}
\centering
    \setlength\arrayrulewidth{2pt}
    \arrayrulecolor{yellow}
    \def\clinecolor{\hhline{>{\arrayrulecolor{yellow}}--%
                            >{\arrayrulecolor{gray}}-}}
    \setlength\extrarowheight{2pt}
\begin{tabular}{ c | c | c }%
    \rowcolor{gray}
\multicolumn{2}{c|}{\thead{AAA -- BBBB}}    &                                   \\
    \clinecolor
    \rowcolor{gray}
\color{white} AAA   &   \color{white} BBB   &   \multirow{-2}{*}{\thead{CCC}}   \\
    \hhline{>{\arrayrulecolor{yellow}}-|-|-}
    \rowcolor{gray!25}
aaa                 &   bbb                 &   ccc                             \\
    \hline
    \rowcolor{gray!25}
ddd             &   eee                 &   fff                             \\
    \hhline{|-|-|-|}
    \end{tabular}
\end{frame}
\end{document}

Use booktabs instead – more precisely set \above/belowrulesep to 0pt and use \midrule:

\documentclass[xcolor=table]{beamer}
\usepackage{makecell, multirow}
\usepackage{booktabs}
\renewcommand\theadfont{\normalsize\bfseries\color{white}}
\renewcommand{\theadgape}{}

\begin{document}

\begin{frame}
  \frametitle{test}
  \centering
  \setlength\arrayrulewidth{2pt}
  \setlength\extrarowheight{2pt}
  \setlength\belowrulesep{0pt}\setlength\aboverulesep{0pt}
  \setlength\lightrulewidth{2pt}
  \arrayrulecolor{yellow}
  \begin{tabular}{ c | c | c }%
    \rowcolor{gray}
    \multicolumn{2}{c|}{\thead{AAA -- BBBB}} & \\
    \midrule
    \rowcolor{gray}
    \color{white} AAA & \color{white} BBB & \multirow{-2}{*}{\thead{CCC}} \\
    \midrule
    \rowcolor{gray!25}
    aaa & bbb & ccc \\
    \midrule
    \rowcolor{gray!25}
    ddd & eee & fff \\
  \end{tabular}
\end{frame}

\end{document}