documentclass[12pt,a4paper,ucs,t,german]{beamer}
usetheme{Ophaniel}
usepackage[utf8x]{inputenc}
usepackage{graphicx}
usepackage{eurosym}
usepackage[export]{adjustbox}

title{textcolor{black}{Playa Turquesa - Dominikanische Republik}}

setbeamercolor{background canvas}{bg=}
setbeamertemplate{frametitle}[default][center]

begin{document}

usebackgroundtemplate{%
  parbox[c][paperheight][c]{paperwidth}{centering{includegraphics[scale=1.0]{../../../immobili/Repubblica-Dominicana/playaturquesa/Playa-Turquesa-Long-Logo-2.png}}}%
}

begin{frame}
%setbeamercolor{background}{bg=violet} % just a try, doesn't work
%setbeamercolor{background canvas}{bg=violet} % this doesn't work either
frametitle{fcolorbox{black}{white}{Beispielpreise}}
Frame text here
end{frame}

end{document}

documentclass[t]{beamer}

setbeamerfont{frametitle}{size=fontsize{24}{28},series=bfseries}
setbeamercolor{frametitle}{fg=black}

setbeamercolor{bgcolor}{fg=black,bg=blue!20}
pgfdeclareimage[width=paperwidth]{mybackground}{back.pdf}

setbeamertemplate{background canvas}{%
    begin{picture}(30,273)
     begin{beamercolorbox}[wd=1.1paperwidth,ht=paperheight]{bgcolor}
     end{beamercolorbox}
        put(-310,0){%
            pgfuseimage{mybackground}
        }
    end{picture}
}

begin{document}

    begin{frame}
        test
    end{frame}

    setbeamercolor{bgcolor}{fg=black,bg=red!20}
    begin{frame}
        test
    end{frame}

end{document}

begin{document}
defopacity{0.3}
begin{frame}
first frame
end{frame}

defopacity{1}
begin{frame}
second frame
end{frame}

end{document}