.button, input[type=submit]{
    //default colour
    background:grey;
}

.button-primary{
    background:green;
}

.button-danger{
    background:red;
}

.button, input[type=submit]:not([class^=button-]){
    background:grey;
}

.button-danger, input[type=submit].button-danger {
    background:red;
}

.button-danger{
    background:red !important;
}