Ignored settings in \context X \with {}

[Dan Eble]

\version "2.19.15"

\score {
  \new Staff <<
%{
    % When these are added, the voice settings below are
    % ignored and the resulting warning is cryptic.
    \context Voice = "one" \with { \override NoteHead.color = #red }
    \context Voice = "two" \with { \override NoteHead.color = #blue }
%}
    <<
      \context Voice = "one" \with { \voiceOne } { a' }
      \context Voice = "two" \with { \voiceTwo } { f' }
    >>
  >>
}