It's 2018: you shouldn't be writing vanilla CSS - Example 7

example-7.styl

$colors = 'blue', 'black', 'yellow', 'aqua';
$dark-colors = 'blue', 'black';

for $color in $colors {
    .btn--{$color} {
        background-color: unquote($color);
        
        if $color in $dark-colors {
            color: white;  
        } else {
            color: #222;
        }
    }
}