Hello Paulo Matos,

Here is the example, In that if limit_reached is true then color will be changed to green.

Example :-

<record id="kanban_view" model="ir.ui.view">
    <field name="name">model.model.kanban.view</field>
    <field name="model">model.model</field>
    <field name="arch" type="xml">
        <kanban class="o_kanban_small_column" create="false">
            <field name="limit_reached"/>
            <field name="name"/>
            <templates>
                <t t-name="kanban-box">
                    <div t-attf-class="oe_kanban_color_#{record.limit_reached.raw_value} oe_kanban_card oe_kanban_global_click">
                        <div class="oe_kanban_content">
                            <strong><field name="name"/></strong>
                        </div>
                    </div>
                </t>
            </templates>
        </kanban>
    </field>
</record>

Hope it will works for you.

Thanks,