Why 4-6, 4-7, 7-5 get these colorings? More precisely, what is the difference between [draw=green, fill=green] to [color=green]?
Why the 4-5 arc has a slightly bigger width that 7-6? You can see a faint black outerline in the 4-5 arc.
This is a minimal example with my questions:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows, positioning, trees, mindmap, calc}
\begin{document}
\begin{tikzpicture}
\tikzset{every node/.style={minimum size=5pt, inner sep=0pt, outer sep=0pt, node distance=25pt}}
\tikzset{black circle/.style={every node, shape=circle, draw=black, fill=black, text=white}}
\tikzset{thick edge/.style={-, black, fill=none, thick, text=black}}
\tikzset{thick arc/.style={->, black, fill=none, thick, >=stealth, text=black}}
\node[black circle] (4) {4};
\node[black circle, below=of 4] (5) {5};
\node[black circle, right=of 5] (6) {6};
\node[black circle, above=of 6] (7) {7};
\draw[thick arc] (4) -- (5);
\draw[thick arc, green] (7) -- (6);
\draw[thick arc, color=green] (4) -- (6);
\draw[thick arc, draw=green] (7) -- (5);
\draw[thick arc, draw=green, fill=green] (4) -- (7);
\draw[thick arc, green] (4) -- (5);
\end{tikzpicture}
\end{document}