The accepted answer doesn't include the detailed explanation why the vertical space is here and why it is different in the colored and uncolored version and why the removing of horizontal space (using %) influences the visual vertical space. I try to explain this.
So, the \maketitle macro above starts the horizontal mode (see \noindent) and first "letter" of this mode is the \vtop (the minipage in LaTeX terminology). Then two following spaces are in OP's code. First one after \vtop (in macro, it was corrected by Steven using %) and second one after right brace from the souce line where \maketitle macro is used. After such two spaces the \hrule primitive is read. This primitive invokes \par, because it is read in horizontal mode. So we have:
\vtop{Bla.}\space\space\par
The \par primitive does one \unskip, so we have
\vtop{Bla.}\space
and this material is assembled to the paragraph. The first "letter" \vtop has the width equal to \hsize, so this "letter" fills the whole first line. The space is the break point and the second empty line is created (the Underfull hbox is reported in log file). But if only one space exists after \vtop in horizontal mode, then this space is removed by \par and the whole paragraph has only one line with \vtop as a "letter".
Summary: the paragraph has two lines if there are two spaces at its end. If only one (or none) space is here then only one line paragraph is created.
The second question: why colored version generates different vertical space than uncolored. The uncolored version generates
\vtop{Bla}
and the baseline of this \vtop is equal to the baseline of the text Bla. But colored version generates
\vtop{\special{color setting}Bla}
and the baseline of \vtop goes through the \special. The text Bla is below the baseline and it consists the depth of the \vtop "letter". The second empty line from the two-line paragraph is closer to the text Bla, when this text is below the baseline. Schema:
Uncolored version:
Bla
------------------ baseline of the first line of paragraph
------------------ baseline of the second line of paragraph
---------------------------- hrule
Colored version:
----special------- baseline of the first line of paragraph
Bla
------------------ baseline of the second line of paragraph
---------------------------- hrule
%after\end{minipage}in your\maketitledefinition. – Steven B. Segletes Mar 16 '15 at 12:13\end{minipage}in your definition. You get different spaces as\colordoesn't switch to hmode and so your minipages have different baselines (add a dot before the\begin{minipage}to see it. You can avoid this by using\noindentbefore#1. – Ulrike Fischer Mar 16 '15 at 12:18