A selection of functions that help create contrasting colours.

contrast-functions.scss

//////////////////////////////////////////////////////////////
//  Dark                                                    //
//////////////////////////////////////////////////////////////

// Checks to see if a colour is dark. The threshold defaults
// to 20% if not set.

// Usage: dark($color, [$threshold])

@function dark($color, $threshold: 20%) {
    @if lightness($color) < $threshold {
        @return true;
    }
    @else {
        @return false;
    }
}

//////////////////////////////////////////////////////////////
//  Contrasted                                              //
//////////////////////////////////////////////////////////////

// Creates a contrasting colour depending on whether or not
// it is a dark colour. If mono is set to true, then it
// returns white or black.

// Usage: contrasted($color, [$ammount], [$mono])

@function contrasted($color, $ammount: 10%, $mono: null) {
    @if dark($color) {
        @if $mono == true {
            @return white;
        }
        @else {
            @return lighten($color, $ammount);
        } 
    }

    @else {
        @if $mono == true {
            @return black;
        }
        @else {
            @return darken($color, $ammount);
        } 
    }
}

You could do the dark function in one line:

@function dark($color, $threshold: 20%) {
    @return lightness($color) < $threshold;
}