colorDomain (domain) {
        if (!arguments.length) {
            return this._colors.domain();
        }
        this._colors.domain(domain);
        return this;
    }
Set or get the current domain for the color mapping function. The domain must be supplied as an
array.

Note: previously this method accepted a callback function. Instead you may use a custom scale
set by {@link ColorMixin#colors .colors}.
@memberof ColorMixin
@instance
@param {Array<String>} [domain]
@returns {Array<String>|ColorMixin}