Change label caption color of a single figure

In my point of view \captionsetup{labelfont={color=blue}} etc. in the local environment is sufficient:

\documentclass[12pt,twoside]{report}
\usepackage{xcolor}
\usepackage[labelfont={color=red}]{caption}

\begin{document}

\chapter{Test chapter}

\clearpage

\begin{figure}
\captionsetup{labelfont={color=blue}}
A
\caption{Test figure A}
\end{figure}

\begin{figure}
B
\caption{Test figure B}
\end{figure}

\end{document}

Here's a start. For the caption itself, you could do as the comments suggest and place a \textcolor in the \caption argument. The number (like "1.2" is contained in \thefigure, while the descriptor word "Figure" is kept in \figurename.

kolygr correctly points out that I don't really need to renew \thefigure, because the color change applied before \figurename will carry through. I will leave the code as is though, in case one gets ideas about changing \figurename and \thefigure to two separate colors.

\documentclass[12pt,twoside]{report}
\usepackage{xcolor}
\usepackage{caption}
\usepackage{caption}
\let\svthefigure\thefigure
\let\svfigurename\figurename
\newcommand\figcolor[1]{%
  \renewcommand\thefigure{\bfseries\sffamily\color{#1}\svthefigure}
  \renewcommand\figurename{\bfseries\sffamily\color{#1}\svfigurename}
}
\begin{document}

\chapter{Test chapter}

\begin{figure}
\centering
A
\caption{Test figure A}
\end{figure}
\begin{figure}
\figcolor{blue}
\centering
B
\caption{Test figure B}
\end{figure}
\begin{figure}
\centering
B
\caption{Test figure B}
\end{figure}

\end{document}

kolygr also points out that the above code will retain the color in \ref attributes. That can be avoided by storing the figure markup in a separate markup macro, and \protecting it so that the expansion (with color) doesn't get written to the aux file:

\documentclass[12pt,twoside]{report}
\usepackage{xcolor}
\usepackage{caption}
\usepackage{caption}
\let\svthefigure\thefigure
\let\svfigurename\figurename
\renewcommand\thefigure{\protect\myfigmarkup\svthefigure}
\renewcommand\figurename{\protect\myfigmarkup\svfigurename}
\let\myfigmarkup\relax
\newcommand\figcolor[1]{%
  \def\myfigmarkup{\bfseries\sffamily\color{#1}}%
}
\begin{document}
In Figure~\ref{fg1.2}...

\chapter{Test chapter}

\begin{figure}
\centering
A
\caption{Test figure A}
\end{figure}
\begin{figure}
\figcolor{blue}
\centering
B
\caption{Test figure B}
\label{fg1.2}
\end{figure}
\begin{figure}
\centering
B
\caption{Test figure B}
\end{figure}

\end{document}