Kind of a hack

interpolation.scss

@function colorchoice($choice) {
    @if $choice == "0" {
        @return rgb(202,0,32);
    }
    @if $choice == "1" {
        @return rgb(244,165,130);
    }
    @if $choice == "2" {
        @return rgb(247,247,247);
    }
    @if $choice == "3" {
        @return rgb(146,197,222);
    }
    @if $choice == "4" {
        @return rgb(5,113,176);
    }
}

@mixin choropleth($bucket) {
    .RdBu .q#{$bucket}-5 {
        $base-fill: colorchoice($bucket);
        fill: $base-fill;
        @include transition;
        @include transition-duration(0.25s);
        &:hover {
            fill: darken($base-fill, 10%);
        }
    }
}

@include choropleth("0");
@include choropleth("1");
@include choropleth("2");
@include choropleth("3");
@include choropleth("4");