New environment error (question)

Question

I'm trying to make a "custom" environment for my document, here's what i got

\documentclass[12pt]{article}
\usepackage{color}
\definecolor{01}{RGB}{0,95,200}
\newenvironment{custom}[2]{{\medskip\noindent\bf\color{01} #1}\\{\it #2 }}{}

\begin{document}

\begin{custom}{Observation}
$x\in Z(G)$ si y solo si $xy=yx$, para todo $y\in G$ si y solo si $N(x)=G$, si y solo si \[\frac{o(G)}{o(N(x))}=1=|[x]|.\]
\end{custom}

\end{document}

which had no problems, but in this case, since the text starts with a "$" symbol, i get an error,

Extra }, or forgotten $. [$] Missing } inserted. [\end{custom}]

Which i can avoid by putting everything on {} like

\begin{custom}{a}
{b}
\end{custom}

But I was wondering if there is a way I can modify the \newenvironment to make it work without having to make any changes, thanks in advance.

Answer 1

As far as I understand, the environment body text should be typeset with \it (of course better with \itshape), so it is not the second argument #2 here, which is not needed.

In the given example of the O.P., there is no 2nd argument given, so LaTeX tries to read the $ from the body leading into errors.

Use \bfseries instead of \bf and \textcolor{01}{#1} rather than \color{01} #1.

\documentclass[12pt]{article}
\usepackage{color}
\definecolor{01}{RGB}{0,95,200}
\newenvironment{custom}[1]{{\medskip\noindent\bfseries\textcolor{01}{#1}}%

\itshape  }{%End code -- empty here
}

\begin{document}

\begin{custom}{Observation}
$x\in Z(G)$ si y solo si $xy=yx$, para todo $y\in G$ si y solo si $N(x)=G$, si y solo si \[\frac{o(G)}{o(N(x))}=1=|[x]|.\]
\end{custom}

\end{document}