使用 elsevier 模板，在 cas-sc cls 中，当使用 tikz 绘制一些图片时，为什么默认颜色不是黑色？如何更改或设置？

Question

\documentclass[a4paper,fleqn]{cas-sc}

\usepackage{tikz}
\usetikzlibrary{positioning, shapes.geometric}
\usetikzlibrary{arrows}
\tikzset{label/.style = {color = white, text = black}       }
\tikzset{   yc/.style = {color = white!1, fill = black!25}  }
\tikzset{ kwcy/.style = {anchor = north, text height = 8pt} }



\begin{document}

\begin{tikzpicture}
\usetikzlibrary{arrows,calc}
\tikzstyle{line} = [-,>=stealth]
\tikzstyle{arrow} = [->,>=stealth]
\tikzstyle{arrow2} = [<->,>=stealth]
    \coordinate (Start) at (0, 0);
    \draw (Start) node [anchor = south west, align=left] {Firms};
    \draw (Start) node [anchor = north west, align=left] {Consumer};
    
    \coordinate (P1End) at ($(Start) + (4, 0)$);
    \draw [line] (Start) -- (P1End);
    \fill[black] (P1End) circle (1pt);
    
    \coordinate (price1)  at ($(Start)  + (2.0, 0)$);
    \coordinate (price1o) at ($(price1) + (0, 0.5)$);
    \draw [arrow] (price1o) -- (price1);
    \node [align=center, above] at (price1o) {$p_{a1},p_{b1}$};
    
    \coordinate (buy1)  at ($(price1) + (1.0, 0)$);
    \coordinate (buy1o) at ($(buy1)   + (0, -0.5)$);
    \draw [arrow] (buy1o) -- (buy1);
    \node [align=center, below] at (buy1o) {which\\ product\\ to buy};
    
    \coordinate (discrimination)  at ($(P1End) + (0.8, 0)$);
    \coordinate (discriminationo) at ($(discrimination) + (0, 0.5)$);
    \draw [arrow] (discriminationo) -- (discrimination);
    \node [align=center, above] at (discriminationo) {whether to \\adopt quality\\ discrimination};

    \coordinate (P21Star) at ($(P1End)$);
    \coordinate (P21End)  at ($(P21Star) + (2, 0)$);
    \draw [line] (P21Star) -- (P21End);
    
    \coordinate (P2NoStart)  at ($(P21End) + (1.2,  -1.5)$);
    \coordinate (P2YesStart) at ($(P21End) + (1.2, 1.5)$);
    
    \draw [line] (P21End)--(P2NoStart)  node [midway,  below left] {No};
    \draw [line] (P21End)--(P2YesStart) node [midway,  above left] {Yes};
    
    \coordinate (qNo)  at ($(P2NoStart) + (1,  0)$);
    \coordinate (qNoo) at ($(qNo) + (0, 0.5)$);
    \node [align=center, above] at (qNoo) {$q_{a2},q_{b2}$};
    \draw [arrow] (qNoo) -- (qNo);
    
    \coordinate (pNo)  at ($(qNo) + (2,  0)$);
    \coordinate (pNoo) at ($(pNo) + (0, 0.5)$);
    \node [align=center, above] at (pNoo) {$p_{ao},p_{bo}$,\\$p_{an},p_{bn}$};
    \draw [arrow] (pNoo) -- (pNo);
    
    \coordinate (buy2No)  at ($(pNo)    + (1.5, 0)$);
    \coordinate (buy2Noo) at ($(buy2No) + (0, -0.5)$);
    \draw [arrow] (buy2Noo) -- (buy2No);
    \node [align=center, below] at (buy2Noo) {whether to stay\\or switch};
    
    
    \coordinate (qYes)  at ($(P2YesStart) + (1,  0)$);
    \coordinate (qYeso) at ($(qYes)       + (0, 0.5)$);
    \node [align=center, above] at (qYeso) {$q_{ao},q_{bo}$,\\ $q_{an},q_{bn}$};
    \draw [arrow] (qYeso) -- (qYes);
    
    \coordinate (pYes)  at ($(qYes) + (2,  0)$);
    \coordinate (pYeso) at ($(pYes) + (0, 0.5)$);
    \node [align=center, above] at (pYeso) {$p_{ao},p_{bo}$,\\ $p_{an},p_{bn}$};
    \draw [arrow] (pYeso) -- (pYes);
    
    \coordinate (buy2Yes) at  ($(pYes)    + (1.5, 0)$);
    \coordinate (buy2Yeso) at ($(buy2Yes) + (0,  -0.5)$);
    \draw [arrow] (buy2Yeso) -- (buy2Yes);
    \node [align=center, below] at (buy2Yeso) {whether to stay\\or switch};
    

    \coordinate (P2NoEnd)  at ($(P2NoStart)  + (6,  0)$);
    \coordinate (P2YesEnd) at ($(P2YesStart) + (6,  0)$);
    \draw [arrow] (P2NoStart)  -- (P2NoEnd);
    \draw [arrow] (P2YesStart) -- (P2YesEnd);
    
    \draw ($(P2NoEnd)  + (-0.2,  0.1)$) -- ($(P2NoEnd)  + (-0.2,  -0.1)$);%P2NoRealize
    \draw ($(P2YesEnd) + (-0.2,  0.1)$) -- ($(P2YesEnd) + (-0.2,  -0.1)$);%P2YesRealize
    
    \draw (P2NoEnd)  node [anchor = west, align=left] {Time};
    \draw (P2YesEnd) node [anchor = west, align=left] {Time};
    
    \coordinate (Period10) at ($(Start) + (1.5, -3)$);
    \coordinate (Period12) at ($(P1End) + (0, -3)$);
    \coordinate (Period22) at ($(Period12) + (9, 0)$);
    \draw [arrow2] (Period10) -- (Period12) node [midway, below] {first period};
    \draw [arrow2] (Period12) -- (Period22) node [midway, below] {second period};
    
    
\end{tikzpicture}

\end{document}