Detecting if inside a tikzpicture

\documentclass{article}
\usepackage{tikz}

\makeatletter
\newcommand{\IfInTikzPic}{% https://tex.stackexchange.com/a/121309/4301
  \ifx\pgfpictureid\@undefined
    \expandafter\@firstoftwo
     \else
    \expandafter\@secondoftwo
    \fi
}
\makeatother

\newcommand*{\DrawLine}[1]{%
  \IfInTikzPic
   {\begin{tikzpicture}[remember picture]
    \draw[ultra thick, ->, #1] (0,) -- (1,1) node [right] {output of DrawLine};
   \end{tikzpicture}}%
    {\draw[ultra thick, ->, #1] (0,) -- (1,1) node [right] {output of DrawLine};}%
}

\begin{document}
%%% The commented out code here is to show that \IfInTikzPic works as desired
%%% (in a tikzpicture, outside of a \node).
%%%

\textbf{IfInTikzPic}\par
\IfInTikzPic{inside}{outside}

\begin{tikzpicture}
  \IfInTikzPic{\draw [red, ultra thick]}{\draw [blue, ultra thick]} (0,0) -- (1,0);
\end{tikzpicture}

\medskip% --------------------------
\textbf{DrawLine}: Actual Output\par

\DrawLine{blue}% <---- How do I get this case to work?

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}

\medskip% --------------------------
\textbf{DrawLine}: Desired Output\par

\begin{tikzpicture}
  \DrawLine{blue}
\end{tikzpicture}

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}
\end{document}

---

Or this version which directly forces tikz rather than having an if-then-else construct

\documentclass{article}
\usepackage{tikz}

\makeatletter
\newcommand{\ensuretikz}{% https://tex.stackexchange.com/a/121309/4301
  \ifx\pgfpictureid\@undefined
    \expandafter\tikzify
     \else
    \expandafter\@firstofone
    \fi
}
\makeatother

\def\tikzify#1{\begin{tikzpicture}#1\end{tikzpicture}}

\newcommand*{\DrawLine}[1]{%
    \ensuretikz
    {\draw[ultra thick, ->, #1] (0,) -- (1,1) node [right] {output of DrawLine};}%
}

\begin{document}
%%% The commented out code here is to show that \IfInTikzPic works as desired
%%% (in a tikzpicture, outside of a \node).
%%%




\medskip% --------------------------
\textbf{DrawLine}: Actual Output\par

\DrawLine{blue}% <---- How do I get this case to work?

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}

\medskip% --------------------------
\textbf{DrawLine}: Desired Output\par

\begin{tikzpicture}
  \DrawLine{blue}
\end{tikzpicture}

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}
\end{document}

The following example defines \IfInTikzPic to check the current environment against tikzpicture and define a \tikzstart...\tikzend pair accordingly:

\documentclass{article}

\usepackage{tikz}

\makeatletter
\def\@tikzenvironment{tikzpicture}

\newcommand{\IfInTikzPic}{%
  \edef\tikzstart{%
    \ifx\@currenvir\@tikzenvironment\else
      \noexpand\tikzpicture[remember picture]%
    \fi
  }%
  \edef\tikzend{%
    \ifx\@currenvir\@tikzenvironment\else
      \noexpand\endtikzpicture%
    \fi
  }%
}

\newcommand*{\DrawLine}[1]{%
  \IfInTikzPic
  \tikzstart
    \draw[ultra thick, ->, #1] (0,) -- (1,1) node [right] {output of DrawLine};
  \tikzend%
}
\makeatother

\begin{document}

\textbf{DrawLine}: Actual Output\par

\DrawLine{blue}

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}

\medskip% --------------------------

\textbf{DrawLine}: Desired Output\par

\begin{tikzpicture}
  \DrawLine{blue}
\end{tikzpicture}

\begin{tikzpicture}
  \DrawLine{orange}
\end{tikzpicture}

\end{document}

You could also group the tikzpicture if needed:

\newcommand{\IfInTikzPic}{%
  \edef\tikzstart{%
    \ifx\@currenvir\@tikzenvironment\else
      \noexpand\begin{tikzpicture}[remember picture]%
    \fi
  }%
  \edef\tikzend{%
    \ifx\@currenvir\@tikzenvironment\else
      \noexpand\end{tikzpicture}%
    \fi
  }%
}