\documentclass[a4paper,12pt]{article}
\usepackage{color}
\begin{document}
\pagecolor{black} % Background color
\color{white}     % Text color


\definecolor{cclr}{rgb}{.7,0.0,0.0}
\definecolor{Eclr}{rgb}{.2,0.0,0.8}
\definecolor{Aclr}{rgb}{.5,0.5,0.5}
axial symetry

\begin{equation}
d\Phi = \textcolor{Eclr}{\vec{E}}\cdot
\textcolor{Aclr}{d\vec{A}} = 0
\end{equation}

\begin{equation}
\Phi_{cap} =\int_{cap} \textcolor{Eclr}{\vec{E}}\cdot
\textcolor{Aclr}{d\vec{A}} = 0
\end{equation}

\begin{equation}
d\Phi = \textcolor{Eclr}{\vec{E}}\cdot
\textcolor{Aclr}{d\vec{A}}
=\textcolor{Eclr}{E_{r}(r)} \textcolor{Aclr}{dA}
\end{equation}

\begin{eqnarray}
\Phi_{can} &=&\int_{can} \textcolor{Eclr}{E_{r}(r)}
\textcolor{Aclr}{dA}\nonumber\\
&=&\textcolor{Eclr}{E_{r}(r)}
\textcolor{Aclr}{A_{can}}
=\textcolor{Eclr}{E_{r}(r)}2 \pi r L
\end{eqnarray}

\begin{equation}
\textcolor{cclr}{q_{enc}} =\textcolor{cclr}{\lambda
}L=\epsilon_0 \Phi_{tot} =\epsilon_0
\textcolor{Eclr}{E_{r}(r)}2 \pi r L
\end{equation}

\end{document}