The rgba() function can accept a single hex color as well decimal RGB values. For example, this would work just fine:
@mixin background-opacity($color, $opacity: 0.3) {
background: $color; /* The Fallback */
background: rgba($color, $opacity);
}
element {
@include background-opacity(#333, 0.5);
}
If you ever need to break the hex color into RGB components, though, you can use the red(), green(), and blue() functions to do so:
$red: red($color);
$green: green($color);
$blue: blue($color);
background: rgb($red, $green, $blue); /* same as using "background: $color" */