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}

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

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

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*}
begingroupcolor{blue}    a = b + cendgroup
end{equation*}
begin{align*}
    a - b &= c\
    a-c & b
end{align*}
end{minipage}

end{document}

deffoo#1$$#2!!{defmathdisplay##1{#1$$begingroup#2}}
expandafterfoomathdisplay{#1}!!

longdeffoo#1$$#2!!{defendmathdisplay##1{#1endgroup$$#2}}
expandafterfooendmathdisplay{#1}!!

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

vline

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

vline

begin{minipage}{4cm}
begin{gather*}
begingroupcolor{red}    a = b + cendgroup\
begin{split}
    a - b &= c\
    a-c &= b
end{split}
end{gather*}
end{minipage}

end{document}

documentclass[12pt]{article}
usepackage{amsmath}
usepackage{color}

begin{document}

start with some text.  make it a rather long line.
begin{equation*}
    a = b + c
end{equation*}
some more text.  make this also a rather long line.
begin{equation*}
color{red}    a = b + c
end{equation*}
still more text.  make this also a rather long line.

end{document}