Extend sass 3.3 colour maps with warning and fallback to red colour

_palette.scss

// extended from http://erskinedesign.com/blog/friendlier-colour-names-sass-maps/
@function palette($palette, $tone: 'base') {
  $color: map-get(map-get($palettes, $palette), $tone);
  @if $color == null {
    @warn "Colour undefined: Palette '#{$palette}' doesn't yet define the tone '#{$tone}'!";
    @return #{red};
  }
  @return $color;
}