TikZ，\graph，无法更改颜色

Question

我不明白为什么节点的样式不变。节点 6 的颜色不变，我该如何实现？似乎创建节点后样式就无法更改。

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{ graphs, graphs.standard }
\usetikzlibrary{ calc }

\begin{document}
    \begin{tikzpicture}
        \graph [nodes={fill = blue}, empty nodes] {
            subgraph I_n [clockwise, radius = 2cm, n = 6];
            6 [fill = red];
            7 [at = { ($(1) ! 0.5 ! (4)$) }, fill = red];
        };
    \end{tikzpicture}
\end{document}

Answer 1

这个问题的答案中描述了几种方法：TikZ 3.0：为子图中的特定节点着色。这里有一个可能的解决方案：

\documentclass{article}
% Adapted from https://tex.stackexchange.com/a/269274/    
\usepackage{tikz}
\usetikzlibrary{ graphs, graphs.standard,backgrounds }
\usetikzlibrary{ calc }

\pgfdeclarelayer{foreground}
\pgfsetlayers{main,foreground}

\begin{document}
    \begin{tikzpicture}
        \graph [nodes={fill = blue}, empty nodes] {
            subgraph I_n [clockwise, radius = 2cm, n = 6];
            };
        \begin{pgfonlayer}{foreground}    
            \node[fill = red] at (6) {};
            \node[fill = red] at ($(1) ! 0.5 ! (4)$) {};
        \end{pgfonlayer}
    \end{tikzpicture}
\end{document}