# Decorate path with just a colored thick line

Is it possible to decorate a path with just a thick colored line above or below it? In the following example I faked manually the result, but how can I do it with decorations? In particular I want to be able to choose the color and the line width and use the raise option of the decoration.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (1,1);
\coordinate (v) at ($(B) - (A)$);

\path let \p1 = (v) in ($1/(\x1^2 + \y1^2)^(0.5)*(\y1,-\x1)$) coordinate (p);
\pgfmathsetmacro{\dist}{0.7}
\coordinate (AA) at ($(A) + \dist*(p)$);
\coordinate (BB) at ($(B) + \dist*(p)$);

\draw (A) -- (B);
\draw[thick,orange] (AA) -- (BB);
\end{tikzpicture}
\end{document}


Edit: If it is not possible to get it via decorations is it possible to define a style just for straight lines which allows you to draw two or three colored lines just like here: http://tex.stackexchange.com/a/36858/4011 in the case of circles.

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Do you want this to work with curves and polygons, or just with straight lines? –  Jake Oct 8 '12 at 12:46
For my current application I want it only for straight lines. –  student Oct 8 '12 at 12:50
But if it is possible to do it with decorations, then it should automatically work with arbitrary path's...? –  student Oct 8 '12 at 12:51
Take a look at: tex.stackexchange.com/q/29991/86 Is that the sort of thing you are after? –  Loop Space Oct 8 '12 at 12:53
@AndrewStacey Thanks, but I think this doesn't really help. I want to know how to do it with decorations and the inside-outside problem doesn't matter in my case... –  student Oct 8 '12 at 13:00

Will using double help here?

\documentclass{standalone}
\usepackage[svgnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
\coordinate (A) at (0,0);
\coordinate (B) at (1,1);
\coordinate (v) at ($(B) - (A)$);

\path let \p1 = (v) in ($1/(\x1^2 + \y1^2)^(0.5)*(\y1,-\x1)$) coordinate (p);
\pgfmathsetmacro{\dist}{0.7}
\coordinate (AA) at ($(A) + \dist*(p)$);
\coordinate (BB) at ($(B) + \dist*(p)$);
\draw[line width=5pt,orange,double=black,double distance=1pt] (AA) -- (BB);

\begin{scope}[xshift=4cm]
\draw[very thick, orange,double=black,double distance=1pt] (-0.1,5) .. controls (-4,-2) and (4,-2) .. (0.1,5);
\end{scope}

\end{tikzpicture}
\end{document}


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Thanks. The problem with double is that then I get the colored line on both sides. –  student Oct 8 '12 at 16:13