Perform the following steps to make an RPC call to the server and display the result in a tooltip:

1. Add the willStart method and set colorGroupData in the RPC call:

  willStart: function () {
        var self = this;
        this.colorGroupData = {};
        var colorDataDef = this._rpc({
            model: this.model,
            method: 'read_group',
            domain: [],
            fields: ['color'],
            groupBy: ['color'],
        }).then(function (result) {
            _.each(result, function (r) {
                self.colorGroupData[r.color] = r.color_count;
            });
        });
        return $.when(this._super.apply(this, arguments), colorDataDef);
    },

2. Update _renderEdit and set up a bootstrap tooltip on pills:

    _renderEdit: function () {
        this.$el.empty();
        var pills = qweb.render('FieldColorPills', {widget: this});
        this.$el.append(pills);
        this.$el.find('[data-toggle="tooltip"]').tooltip();
    },

3. Update the FieldColorPills template and add the tooltip data:

<t t-name="FieldColorPills">
    <t t-foreach="widget.totalColors" t-as='pill_no'>
        <span t-attf-class="o_color_pill o_color_#{pill_no} #{widget.value === pill_no and 'active' or ''}"
        t-att-data-val="pill_no"
        data-toggle="tooltip"
        data-placement="top"
        t-attf-title="This color is used in #{widget.colorGroupData[pill_no] or 0 } books."
        />
    </t>
</t>

Update the module to apply the changes. After the update, you will be able to see a tooltip on the pills, as shown in the following screenshot: