A \textcolor{blue}{blue remark}, a \textcolor{red}{red warning}, and
colour inside mathematics:
\[
\textcolor{blue}{a^2} + \textcolor{red}{b^2} = c^2 .
\]
And \emph{tints}, \verb|blue!20!white| and \verb|red!20!white|, shading a
picture of a wave tunnelling through a barrier.
\begin{center}
\begin{tikzpicture}[x=1cm, y=1cm]
% where the wave may travel, and the barrier it tunnels through
\fill[blue!20!white] (0,0) rectangle (3,1.7);
\fill[red!20!white] (3,0) rectangle (4.2,1.7);
\fill[blue!20!white] (4.2,0) rectangle (7,1.7);
% the potential, and the wave's energy
\draw[red, thick] (0,0.25) -- (3,0.25) -- (3,1.35) -- (4.2,1.35) -- (4.2,0.25) -- (7,0.25);
\draw[black!50, dashed] (0,0.8) -- (7,0.8) node[right] {$E$};
% the wave function: oscillating, decaying inside the barrier, and out again smaller
\draw[blue, thick, smooth, samples=90, domain=0:3] plot (\x, {0.8+0.4*sin(deg(4*\x+2.137))});
\draw[blue, thick, smooth, samples=30, domain=3:4.2] plot (\x, {0.8+0.4*exp(-1.6*(\x-3))});
\draw[blue, thick, smooth, samples=90, domain=4.2:7] plot (\x, {0.8+0.059*sin(deg(4*(\x-4.2))+90)});
\node[blue] at (1.1,1.45) {$\psi$};
\node[red] at (3.6,1.52) {$V_0$};
\end{tikzpicture}
\end{center}