pgfkeys{/IMMstyle/.style={
         inputnode/.style={shape=ellipse,draw,inner sep=2pt,minimum size=5.0mm},
         definecolor{dpred}{rgb}{0.50,0.35,0}
        }}

tikzset{
    define color/.code 2 args={
        definecolor{#1}{rgb}{#2}
    }
}

documentclass{book}
usepackage{tikz}
usetikzlibrary{shapes}

begin{document}

tikzset{
    define color/.code 2 args={
        definecolor{#1}{rgb}{#2}
    },
    /IMMstyle/.style={
         inputnode/.style={
            shape=ellipse,
            draw,
            inner sep=2pt,
            minimum size=5mm
         },
         define color={dpred}{0.80,0.35,0}
    }
}

begin{tikzpicture}
node [/IMMstyle,inputnode,fill=dpred] {x};
end{tikzpicture}

end{document}

begin{tikzpicture}
definecolor{dkblue}{rgb} {0.00,0.33,0.68}
tikzset{
    myStyle/.style = {draw=dkblue, very thick, rectangle}
}

node [myStyle] {x};
end{tikzpicture}