how to color a repeat bar line?

[Patrick Karl]

I would like to color the initial repeat bar line of a volta.  I tried the 
following:

\version "2.14.1"
\relative g' {
        \repeat volta 2 { a b c d }
        \break
        \once \override Staff.BarLine #'color = #red
        \repeat volta 2 { d c b a }
}

but this colors not only the initial bar line of the 2nd repeat, but also the 
final bar line of the 1st repeat (which is an interesting interpretation of 
"\once").  Is there a way to only color the initial bar line of the 2nd repeat?