tikz picture in eso-pic

Question

I construct a macro named \drawline to draw a picture by tikz package on every page(by eso-pic package). When I call this macro twice in order to pain two lines in different places and with different color(red and blue), the typeset gives two lines with the same color(red - the color given in the last call). Why and how to deal with this problem? PS：In the following MWE, only one arg is given. In fact, there are several other args other than for tikz, so \pgfkeys must be used.

\documentclass{article}
\usepackage{tikz,eso-pic,xparse,picture}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newcommand{\drawlinecenter}{%
  \AddToShipoutPictureBG{%
    \AtTextCenter{%
      \put(\xx,\yy){%
      \begin{tikzpicture}
        \draw[center,line width=6pt]
          (0,0) -- (0,1);
      \end{tikzpicture}
    }}}}
%    
\newcommand{\drawlineupperleft}{%
  \AddToShipoutPictureBG{%
    \AtTextUpperLeft{%
      \put(\xx,\yy){%
      \begin{tikzpicture}
        \draw[upperleft,line width=6pt]
          (0,0) -- (0,1);
      \end{tikzpicture}
    }}}}
%      
% main macro:
\NewDocumentCommand{\drawline}{O{}m}{% #1-optional style arg. #2-position
  \tikzset{%
    x/.code=\def\xx{##1},x/.default=0pt,
    y/.code=\def\yy{##1},y/.default=0pt,
    #2/.style={x,y,#1}
  }
  \pgfkeys{/tikz/.cd,x,y,#1}
  \csname drawline#2\endcsname
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\drawline[color=blue,x=1cm]{center}
\drawline[color=red,x=10cm]{upperleft}
firstpage\clearpage second page\clearpage third page
\end{document}

Answer 1

You are overwriting the line color. I would probably just store these things in styles. Apart from solving the issue you will be more flexible, i.e. could add e.g. dashed and so on.

\documentclass{article}
\usepackage{tikz,eso-pic,xparse}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newcommand{\drawlinecenter}{%
  \AddToShipoutPictureBG{%
    \AtTextCenter{%
      \begin{tikzpicture}
        \draw[center,line width=6pt]
          (0,0) -- (0,1);
      \end{tikzpicture}
    }}}
%    
\newcommand{\drawlineupperleft}{%
  \AddToShipoutPictureBG{%
    \AtTextUpperLeft{%
      \begin{tikzpicture}
        \draw[upperleft,line width=6pt]
          (0,0) -- (0,1);
      \end{tikzpicture}
    }}}
%      
% main macro:
\NewDocumentCommand{\drawline}{O{}m}{% #1-optional style arg. #2-position
  \tikzset{#2/.style={#1}}
  \csname drawline#2\endcsname
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\drawline[color=blue]{center}
\drawline[color=red]{upperleft}
firstpage\clearpage second page\clearpage third page
\end{document}

Answer 2

Thanks to marnot, the following may be a solution(in which pgf's /.expanded used):

\documentclass{article}
\usepackage{tikz,eso-pic,picture}

\begin{document}
% main macro:
\NewDocumentCommand{\drawline}{O{}m}{% #1-optional style arg. #2-position
  \pgfkeys{%
    x/.store in=\x,x/.default=0,
    y/.store in=\y,y/.default=0,
    color/.store in=\cl,color/.default=blue,
    tc/.code n args={3}{%
        \AddToShipoutPictureBG{%
          \AtTextCenter{%
             \put(##1,##2){%
               \tikz\draw[draw=##3,line width=6pt](0,0) -- (0,1);
            }}}},
    ul/.code n args={3}{%
        \AddToShipoutPictureBG{%
          \AtTextUpperLeft{%
            \put(##1,##2){%
              \tikz\draw[draw=##3,line width=6pt](0,0) -- (0,1);
            }}}},  
   }
   \pgfkeys{x,y,color,#1,#2/.expanded={\x}{\y}{\cl}}
 }
% test
\drawline[color=green,x=-5cm]{tc}
\drawline{tc}
\drawline[color=red,x=0cm,y=-3cm]{ul}

first page\clearpage second page\clearpage third page
\end{document}