Extra space added after equation when it is coloured

Question

For some reason space is added after my equation when I colour it. This does not happen if the colour is added to text.

Is there a simple way to prevent this from happening, like a global setting (I am not interested in manually adjusting the space with \vspace{-1cm})? Perhaps there is a better way to colour my equation (another package, another command, etc), or in other words, am I doing something wrong?

\documentclass[12pt]{standalone}

\usepackage{amsmath}
\usepackage{color}

\begin{document}

\begin{minipage}{4cm}
\begin{equation*}
    a = b + c
\end{equation*}
\begin{align*}
    a - b &= c\\
    a-c & b
\end{align*}
\end{minipage}

\vline

\begin{minipage}{4cm}
\begin{equation*}
\color{blue}    a = b + c
\end{equation*}
\begin{align*}
    a - b &= c\\
    a-c & b
\end{align*}
\end{minipage}

\end{document}

For reference, the closest question I found to this one doesn't seem to solve my problem, ie, adding \fboxsep0pt to my preamble did not change the output.

EDIT Kurt's comment solves part of the problem, but it doesn't work when using for example align (it gives an error):

\begin{align*}
{\color{blue}    a - b &= c}\\
    a-c &= b
\end{align*}

If I try to fix this using the command twice leaving the & outside, the spacing around = is wrong:

\begin{align*}
{\color{blue}    a - b} &{\color{blue}= c}\\
    a-c &= b
\end{align*}

Answer 1

Oh. It's an AMS bug, the alignment is not color safe . Never noticed that before:-) In that case you need explicit grouping (use begingroup rather than bgroup or { as it's rather less intrusive into math spacing)

\documentclass[12pt]{standalone}

\usepackage{amsmath}
\usepackage{color}

\begin{document}

\begin{minipage}{4cm}
\begin{equation*}
    a = b + c
\end{equation*}
\begin{align*}
    a - b &= c\\
    a-c & b
\end{align*}
\end{minipage}

\vline

\begin{minipage}{4cm}
\begin{equation*}
\begingroup\color{blue}    a = b + c\endgroup
\end{equation*}
\begin{align*}
    a - b &= c\\
    a-c & b
\end{align*}
\end{minipage}

\end{document}

---

It's actually a bit harsh to call it an AMS bug, the same effect is seen with \[ in LaTeX however rather than grouping each case, this is probably the correct fix (anywhere in the preamble after loading amsmath. I'll ping @barbarabeeton

\def\foo#1$$#2!!{\def\mathdisplay##1{#1$$\begingroup#2}}
\expandafter\foo\mathdisplay{#1}!!

\long\def\foo#1$$#2!!{\def\endmathdisplay##1{#1\endgroup$$#2}}
\expandafter\foo\endmathdisplay{#1}!!