Works with this changes

interpolation.scss

$colors: rgb(202,0,32), rgb(244,165,130), rgb(247,247,247), rgb(146,197,222), rgb(5,113,176);

@mixin choropleth($bucket) {
    .RdBu .q#{$bucket}-5 {
        $base-fill: nth($colors, $bucket);
        fill: $base-fill;
        &:hover {
            fill: darken($base-fill, 10%);
        }
    }
}

@include choropleth(1);
@include choropleth(2);
@include choropleth(3);
@include choropleth(4);
@include choropleth(5);