Variables

$rainbow: red orange yellow green blue indigo violet;

You could set individual variables for each color as well. You would still pass them all as a single argument, or join them into a single variable before passing, as you see fit.

Function

// Returns a striped gradient for use anywhere gradients are accepted.
// - $position: the starting position or angle of the gradient.
// - $colors: a list of all the colors to be used.
@function stripes($position, $colors) {
  $colors: if(type-of($colors) != 'list', compact($colors), $colors);
  $gradient: compact();
  $width: 100% / length($colors);

  @for $i from 1 through length($colors) {
    $pop: nth($colors,$i);
    $new: $pop ($width * ($i - 1)), $pop ($width * $i);
    $gradient: join($gradient, $new, comma);
  }

  @return linear-gradient($position, $gradient);
}

Use

.rainbow {
  @include background-image(stripes(left, $rainbow));
}

Demo

Here is the result:

Jina has also posted a demo and explanation on CodePen.

(The real lesson here is that all the colors of the rainbow are acceptable CSS color keywords. Go forth and queer the web.)