`color:value` warning when using `sass`

app/assets/stylesheets/style.css.scss has the following definitions:

@each$color in red, purple, blue, green, yellow, orange {
  .c_#{$color}{
    color:$color;
  }
}

The style is applied, but the following warning appears in the terminal:

You don't mean to use the color value 'blue' in interpolation here.
It may end up presented as #0000ff, which will like produce invalid CSS.
Always quote color names when using them as strings (for example, "blue").
If you really want to use the color value here, use'"+$color'.

As stated in the warning, we modified it as follows, but the style is not applied and it is invalid.

@each$color in red, purple, blue, green, yellow, orange {
  .c_#{$color}{
    "" +$color;
  }
}

How do I fix this?

environment:Rails 4.2.10