Tukukkk

Below code use positioning packet to put ruler line between two color lines.
But the result not as expected!

documentclass[border=1pt,varwidth=5cm]{standalone}

usepackage{tikz}

usetikzlibrary{positioning}

begin{document}

newcommandcolorrulemix[1]{textcolor{#1!40!gray}{rule{0.5cm}{0.5cm}} }

newcommandcolorrule[1]{textcolor{#1}{rule{0.5cm}{0.5cm}} }

begin{tikzpicture}[node distance=0]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    node[below=0.1 of P4] {rule{4.5cm}{1pt}};

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2 of Pi] (Qi) {colorrulemix{name}};

    }

end{tikzpicture}

end{document}

The main reason why your approach does not quite work is that the nodes have some standard dimensions, most notably inner sep, which distorts the (vertical) position of your bar node. If you remove the inner sep, the bar will sit right below the baseline that is in the middle. You could then shift it down by half its thickness. It is, however, easy enough to draw a perfectly (I hope;-) centered bar with TikZ methods.

documentclass[border=1pt,varwidth=5cm]{standalone}

usepackage{tikz}

usetikzlibrary{positioning}

begin{document}

newcommandcolorrulemix[1]{textcolor{#1!40!gray}{rule{0.5cm}{0.5cm}} }

newcommandcolorrule[1]{textcolor{#1}{rule{0.5cm}{0.5cm}} }



Your picture:\

begin{tikzpicture}[node distance=0]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    node[below=0.1 of P4] {rule{4.5cm}{1pt}};

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2 of Pi] (Qi) {colorrulemix{name}};

    }

end{tikzpicture}



Your picture with texttt{inner sep=0pt}:\

begin{tikzpicture}[node distance=0]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    node[below=0.1 of P4,inner sep=0pt] {rule{4.5cm}{1pt}};

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2 of Pi] (Qi) {colorrulemix{name}};

    }

end{tikzpicture}



Your picture with texttt{inner sep=0pt} and a cross--check:\

begin{tikzpicture}[node distance=0]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    node[below=0.1 of P4,inner sep=0pt] {rule{4.5cm}{1pt}};

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2 of Pi] (Qi) {colorrulemix{name}};

    }

    path (Q1) -- (P1) coordinate[midway] (aux) ;

    draw[red](aux) -- ++ (4,0);

end{tikzpicture}



An alternative with really centered line:\

begin{tikzpicture}[node distance=0]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2 of Pi] (Qi) {colorrulemix{name}};

    }

    path (Q4) -- (P4) coordinate[midway] (aux) ;

    draw[line width=1pt]([xshift=-2.25cm]aux) -- ++ (4.5,0);

end{tikzpicture}

end{document}

First of all, it is useless to specify the node distance= 0 if you specify the < shift part > of the keys below of= as I explained here : TikZ: How does global/local [node distance] work?

Then, do not confuse the points of the path and the nodes. Nodes are not part of the path itself and have additional parameters such as the distance between the text and the border (inner sep) and the distance between the border and the outside (outer sep): read pages 218 and 219 of the manual 3.0.1a.

Thus, it is sufficient to specify these parameters globally

begin{tikzpicture}[inner sep=0pt,outer sep=0pt] 

and do not forget that when you go down from 0.2, you have omitted to count the line thickness which is 1pt : rule{4.5cm}{1pt}

It is therefore necessary to add this length.

node[below=0.2cm+1pt of Pi] (Qi) {colorrulemix{name}

The result and the final code are:

documentclass[border=1pt,varwidth=5cm]{standalone}

usepackage{tikz}

usetikzlibrary{positioning}

begin{document}

newcommandcolorrulemix[1]{textcolor{#1!40!gray}{rule{0.5cm}{0.5cm}} }

newcommandcolorrule[1]{textcolor{#1}{rule{0.5cm}{0.5cm}} }

begin{tikzpicture}[inner sep=0pt,outer sep=0pt]

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node (Pi) at (i*0.6cm,0) {colorrule{name}};

    }  

    node[below=0.1 of P4] {rule{4.5cm}{1pt}};

    foreach name [count=i] in {{red},{orange},{yellow},{green},{cyan},{blue},{purple}} {

        node [below=0.2cm+1pt of Pi] (Qi) {colorrulemix{name}};

    }

end{tikzpicture}

end{document}

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