colorAccessor (colorAccessor) {
        if (!arguments.length) {
            return this._colorAccessor;
        }
        this._colorAccessor = colorAccessor;
        return this;
    }
Set or the get color accessor function. This function will be used to map a data point in a
crossfilter group to a color value on the color scale. The default function uses the key
accessor.
@memberof ColorMixin
@instance
@example
// default index based color accessor
.colorAccessor(function (d, i){return i;})
// color accessor for a multi-value crossfilter reduction
.colorAccessor(function (d){return d.value.absGain;})
@param {Function} [colorAccessor]
@returns {Function|ColorMixin}