odoo18/addons_extensions/sign/static/src/scss/iframe.scss

@for $size from 2 through length($o-colors) {
    // Note: the first color is supposed to be invisible so it's ignored
    .o_color_responsible_#{$size - 1} {
        background-color: nth($o-colors, $size);
        opacity: 0.9;
    }
}