Problem with \specialcomment-command and equation/align-environments

Question

I'm using the specialcomment-command from the comment package around equation and align environments. This leads to an unwanted blank line after the math environment. When just using the simple includecomment-command, the problem does not show, neither when my special comment just encloses "normal" text. In my example, this doesn't look so bad. But in my actual document there are other settings so it just doesn't look good. Changing the spacing by \specialcomment{mycomment2}{\color{red}}{\color{black}\vspace{-4ex}} or something like that messes with the normal commented text. How can I fix this?

\documentclass{article}
\usepackage{blindtext}
\usepackage{xcolor}
\usepackage{comment}
\includecomment{mycomment1}
\specialcomment{mycomment2}{\color{red}}{\color{black}}
\begin{document}
\begin{mycomment1}
\begin{equation}
x=y
\end{equation}
\end{mycomment1}

\blindtext

\begin{mycomment2}
\begin{equation}
x=y
\end{equation}
\end{mycomment2}

\blindtext

\begin{mycomment2}
Just some normal text, no problem there.
\end{mycomment2}

\blindtext
\end{document}

---

Edit: It seems that a bit of code I left out in order to make the problem as simple as possible is relevant after all. My code regarding the equations is actually like this:

\begin{mycomment2}
\begin{equation}
x=y
\end{equation}
\end{mycomment2}
%
\begin{flushleft}
Text
\end{flushleft}
\blindtext

In this case there is an extra blank line even though there are none in the code.

Answer 1

This isn't actually caused by the comment package, it's the colour change, as you can see from this example.

\documentclass{article}
\usepackage[english]{babel}
\usepackage{blindtext}
\usepackage{xcolor}
\begin{document}
\begin{equation}
x=y
\end{equation}

\blindtext

\color{red}
\begin{equation}
x=y
\end{equation}
\color{black}

%
\blindtext

\color{red}
Just some normal text, no problem there.
\color{black}

\blindtext
\end{document}

To eliminate the problem with blank lines, replace \color{black} with \vskip0pt\color{black}\noindent. However, the colouring macros will still cause trouble if a mycomment2 is followed by an environment that begins a new line, and I am not aware of any general solution. @David Carlisle gives a good explanation of the problem and a workaround here.

Answer 2

If the special comment mycomment2 always constitutes a separate paragraph try:

\makeatletter

\specialcomment{mycomment2}{%
  \par\begingroup\color{red}%
}{%
  \par\global\@tempskipa\lastskip\removelastskip\endgroup\vskip\@tempskipa%
}

\makeatother

Unfortunately, using \color can easily degrade vertical spacing. The critical internal macros are \set@color and \reset@color. Since they are driver-dependent it might be difficult to find a general fix to the vertical spacing problems induced by them.