While it was possible to remove the spacing between paragraphs, I could not figure out how to remove the spacing between lines (line breaks / new lines).
As shown in the first section of the following example \parskip allows to have the tikzpictures without a gap inbetween, but both explicit newlines (\\) in the second section, as well as automatic line breaks as seen in the third section introduce a gap.
\documentclass[a4paper,parskip]{scrartcl}
\usepackage{tikz}
\begin{document}
\setlength{\parskip}{-1pt}
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newcommand{\card}[1] {%
\begin{tikzpicture}%
\fill[#1] (0,0) rectangle (5 cm,2 cm);
\end{tikzpicture}%
}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\card{black}%
\card{red}%
\card{blue}%
\card{red}%
\card{blue}%
\card{black}%
\card{red}%
\vspace{.5cm}
\card{red}%
\card{blue}%
\card{black}\\%
\card{blue}%
\card{black}%
\vspace{.5cm}
\card{red}\hspace{0pt}%
\card{blue}\hspace{0pt}%
\card{black}\hspace{0pt}%
\card{blue}\hspace{0pt}%
\card{black}\hspace{0pt}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\end{document}
Adjusting \baselineskip had no effect....

parskipbut setting\parskipto a value <= 0 make sense? Wouldn't it be better to remove optionparskip? – Schweinebacke Jul 26 '17 at 13:19parskip. And the vertical space between paragraphs in this case is the same a in the paragraphs:\lineskip. Using optionparskipbut then setting\parskipto something <= 0 (or even something <<.5\baselineskip) does not make sense. – Schweinebacke Jul 26 '17 at 16:23